My wife and I were friends in college for quite some time before we started dating. I knew her brother, we ran in the same crowd, I'd been to their house for dinner and that sort of thing. We were well acquainted. She and I occasionally hung out together just the two of us, especially during the summer when there is basically nobody on campus except the few stragglers doing a summer quarter or with a summer job. So we'd meet after work and grab dinner together at the cafeteria, hang out for an hour or two in the evenings just talking about our day. I had zero intentions towards her, it was just companionship.

She came to me one day and wanted to go for a walk. There was a park nearby that we'd been to before a few times, so we went there. Then she started to cry. I thought something really horrible had happened to her, so I asked what was wrong. She said she wanted a hug first, before she would tell me. So I gave her a nice squeezy hug and told her I'd help her however I can, but I need to know what's wrong. Then she apologized over and over again, saying she was so sorry for betraying me, and for ruining everything, she tried so hard to just be friends but she's desperately in love with me and she feels like the scum of the earth for not keeping things platonic, like I wanted. I was completely surprised, because I had absolutely no idea that she felt anything at all for me. Not a clue. And I sure as hell didn't know what to say.

So while I sat there speechless, she spilled her guts, telling me all about everything we'd done together, and everything she'd been thinking, and feeling, and doing, and how wretched she felt. And it made me feel like I was the scum of the earth for making her feel that way. I liked her. I cared about her. Not romantically, but I really didn't like seeing her hurt. At all. It hurt me to see her hurt. So I told her it was fine, she shouldn't apologize, it happens, and isn't always in our control. But she said she couldn't keep doing this, we needed to stop hanging out and being friends, she needed time and distance to get over me.

I think it was two days later the next time I saw her, and I told her that I didn't like avoiding her, I didn't like seeing her hurt or thinking of her being hurt, so we should commit to each other.

Two years later we got married.

tl;dr Friend felt like she betrayed our friendship by developing feelings for me. I felt bad, so I married her.