Reason Number One: Your wife. Julie and I have never been what you'd call close friends. She's quite a few years older than me, so we didn't hang around with the same crowd when we were younger. But I've always liked Julie, and living in a small town, it's inevitable that our paths will cross from time to time. Like they did last weekend. I told you I was going to Maria's hen party and presumably you knew Julie would be there, too. But you didn't tell me that. You didn't think to warn me that I could end up sitting next to your wife in the Indian restaurant.
Julie was quiet that night and I struggled to make conversation. I asked if she'd had a nice time in Dublin and she frowned slightly, asking me what I meant, because she'd never been to Dublin. I stammered that I thought Maria had said something about the family going there on holiday and apologised because I'd obviously misunderstood.
"Chris was in Dublin on business recently," said Julie.
"Oh was he?" I said. "That's a coincidence."
Someone sitting across the table asked Julie how her mum was after the operation and I said that I hadn't known she was ill. Julie said there was no reason why I should. She said it had been a strain, organising someone to look after the kids while she went back and forth to the hospital and made sure her dad was eating properly.
"What abut Chris?" Maria asked. "Surely he must have been able to help with the kids at least?"
Julie shook her head and said you'd had to go to Brussels on business, and when you came back you had to put in a lot of extra hours at the office to catch up on all the work that had piled up while you were away.
And then she went very quiet again, and when someone asked her if she was feeling OK, she started crying. I gave her my napkin to wipe her eyes, and when she took it she apologised for being stupid and told everyone to ignore her because she didn't want to ruin Maria's night.
We were going on to a club after the meal, but Julie said she wasn't feeling great and would get a taxi home. Katie waited with her until the taxi came and I could see the two of them standing outside in the cold night air, their heads almost touching. Katie had her arm wrapped around Julie's shoulder and when the taxi came they hugged and Katie stood on her own for a while before coming back inside.
"Is Julie OK?" I asked, hoping my voice didn't sound as hollow to everyone else as it did to me.
"Not really," Katie replied, quite curtly, I thought. But maybe I'm just paranoid.
"Is it Chris?" Maria asked, adjusting the fluffy pink tiara that the girls had made her wear. Katie just shrugged and went to pay the bill.
"I don't know why she puts up with him," one of the others said. "He treats her like dirt and then begs her to forgive him."
"That's the trouble with good-looking guys," said Maria. "They think they only have to turn on the charm and every woman in the country will fall down and kiss their feet."
"Julie deserves better," someone else said. "If I found out my Liam was having an affair that would be the end of it."
"But we don't even know if Chris is having an affair," I said, springing to your defence. "Julie never said anything about an affair, and if Katie knows what's going on, she's not telling."
"No," said Maria. "Katie won't tell. She's a good friend. Loyal and trustworthy. Like a friend should be."
Maybe it was my imagination, but it seemed like they were all looking straight at me.
Reason Number Two: Your kids. I recognised them right away. Lewis has your dark, curly hair and Emily had on the same green top she's wearing in the photograph you keep in your wallet. They were arguing over whose turn it was to hold the lead attached to a Jack Russell terrier.
"Auntie Fiona, tell Lewis it's my turn to take Oscar," Emily begged.
"It's mine," Lewis protested. "You had him for much longer than me before we reached the park.
"I think we should let Oscar off his lead for a while," the woman called Fiona replied. "He can have a run around on his own while we sit here and rest."
I buried my face in my magazine I was pretending to read. I could have - probably should have - got up from the bench and walked away. But I didn't.
"I can't wait for my birthday," said Emily, her voice shrill with excitement. "All my friends are coming to the party. And you'll be there, won't you, Auntie Fiona?"
"I certainly will," Fiona smiled at the little girl. "I'm looking forward to it."
"Gran's coming too, now that she's better," Emily said. "And Grandpa, and Mummy and Daddy, of course. How many is that?" she asked, holding up her hands to start counting.
"Daddy might not be there," said Lewis, interrupting her before she'd even got to six. "He didn't come to mine, remember. He had to work."
"But my party's on a Saturday," said Emily. "Daddy doesn't work on Saturdays."
"My party was on a Saturday, too," said Lewis. His voice had gone all flat. "But daddy couldn't manage it."
"Auntie Fiona -" the little girl looked up into the woman's face - "tell him. Tell Lewis that my daddy will come to my birthday party on Saturday."
Fiona put her arms around the little girl and hugged her tight. "I'm sure your daddy will be there if he can, darling," she said, looking over the top of Emily's head. "Now, why don't we go and find Oscar and you can hold his lead until we get to the main road."
I watched them walk away before I stuffed the magazine in my bag and went home to my empty house.
Reason Number Three: My friends and family. They all know, of course. Even the ones I haven't told. Don't panic. Most of them don't know who you are. Just that you exist. And that you're married. I didn't tell them that either, but they sussed it out themselves. Why else do we never go out together to work dos or family gatherings? Why else would I turn up by myself to my cousins wedding?
"On your own?" my Aunt Sarah asked with a sly grin on her face. "When are we going to meet this mysterious man of yours?"
"There's nothing very mysterious about him, I'm afraid." I forced myself to laugh, and held her gaze. "He travels a lot in his job, that's all."
But we both knew neither of us was being fooled.
My mum simply refuses to acknowledge your existence. She's given up inviting me for Sunday lunch, because when she did I spent most of the time checking for text messages. It used to be she'd ask me about my social life, what I'd been doing and who I'd been seeing, but nowadays, she doesn't talk to me about anything, except the weather and my work. When I told her I was going to Brussels at the beginning of last month, she didn't ask who I was going with. She didn't ask if I'd had a nice time when I came home either.
If only my sister would play by mum's rules. But she's never been one for holding back.
"You're a fool, Karen. You know that, don't you? He's using you. And when he's had enough of you he'll ditch you and move on to someone else."
"You know nothing about him," I snarled at her.
"I know he's got a wife and kids. Have you thought about what he's doing to them?"
"Of course I've thought about it," I replied. "Neither of us wants to hurt them. That's why he hasn't left them. He's waiting until the time's right."
"And when will that be?" asked Debbie. "His kids are only little. When will the time ever be right?"
"I don't mind waiting," I said. "I don't want to hurt them either."
My mother came into the kitchen at that point, quickly turned and went back out again.
"And what about mum?" Debbie asked me. "Have you even thought about what this is doing to her?"
"It's got nothing to do with her," I replied, weakly.
Debs opened her mouth but seemed to think better of it and closed it again. That was five weeks ago and we haven't spoken since.
Reason Number Four: You. You are so good-looking. And you are very charming. When I first met you at Maria's party, I held my breath. It was Julie who introduced us. Julie and I hadn't seen each other for a couple of years and you fetched drinks for us while we caught up. When Julie got a message from the babysitter saying that the kids wouldn't settle, you sighed and said you'd call a taxi, and when Julie suggested you stay and enjoy yourself, you didn't put up much of a fight. I remember thinking at the time, if my brother-in-law had done that to Debs I'd have punched him in the balls!! I also remember the disappointment in Julie's face when she left the party alone.
We shared a taxi home that evening, and when it pulled up outside my house you asked for my mobile phone so you could tap in your number. You were so sure I'd call. I promised myself I wouldn't. My resolve lasted three days.
In the beginning you were kind, and sweet, and generous. We didn't speak much about Julie and the children then, but gradually I began to think about how our relationship was affecting them. Some might say what I really cared about was how they were affecting my relationship with you. Maybe they're right.
You never said anything nasty about Julie. You were far too clever for that. You said that you had loved her once, but your love had grown cold. Although you still cared about her, and the children, of course. You didn't want to hurt them. I loved you and respected you for that.
You took it for granted that I'd always be there for you, waiting at the end of a phone, or sitting alone in the furthest corner of the supermarket car park on cold winter nights, wondering if you'd actually turn up this time. You promised it wouldn't be like this forever. Only until the kids were old enough to understand.
We went to Brussels and you spent almost the entire flight chatting up the young Irish woman sitting across the aisle.
And when I told you I'd been invited to Maria's hen party you didn't try to persuade me not to go. Even though you knew Julie was going to be there. You certainly didn't think it worth mentioning there was a possibility I might end up sitting next to your wife. On Tuesday, you sent a text saying you were coming over this Saturday, as Julie was taking the kids out for the day.
Reason Number Five: Me. I used to like myself. More than that, I used to respect myself. I used to be like Katie. Loyal and trustworthy. Like a friend should be. I used to visit my family for Sunday lunch and help mum with the washing up while Debs snoozed on the chair, dribbling. I never went as long as five weeks without speaking to my sister.
These are reasons for ending the affair. But there's another one. Maybe the most important one of all: It's a little girl's birthday party on Saturday and all she wants is for her daddy to be there!