I had a serious girlfriend at the end of and just after college. I loved her, but for reasons I won't go into I could not live with and marry her. I felt really bad breaking up with her, but I was moving abroad and it was time.

A couple months later one of my best friends told me they were going to start dating. He wasn't asking, he was informing so that I didn't hear it from someone else first. He certainly didn't "owe me" not to date her.

It made sense to me that they would hit it off. He is a great guy. She is a great girl, just not quite right for me. They were both single. And since I was the one who called it quits, it would have been a serious dick move to be pissed that they would date.

Long story less long, the ended up getting married and have been together ever since. I'm completely happy for them.