<human>: Can you provide a brief overview of the text?

My best friend and I have been close since Middle School. 

He was with the same girl since our Senior year of college.  He talked about being unsure of the relationship and feeling like he wasn't really in love with her, but they stayed together because in the end he would always say how there really wasn't anything wrong with her and it was him.

Eight or nine months ago he met this girl who was in kind of the same situation.  She told him her relationship was coming to an end, she and her boyfriend were pretty much tying up the loose ends that come with living together.  There was pretty obvious chemistry and for whatever reason he told her he and his girlfriend were in the same place.

Anyway, they kissed one night and that turned into a full blown affair.  It didn't last long before they both were out of their previous relationships but it still kind of bothers me.

All things considered, I've never seen my friend happier.  He's "in love" like I've never seen and I don't see any signs of him cheating on her or her cheating on him.  Everyone that meets them and doesn't know about what happened thinks they're awesome together.

Anyway, I know it's not my place to judge but I can't help but be a little weirded out by the relationship.  I don't plan on doing anything or saying anything to sabotage it or anything like that, I guess I just need some feedback or some stories to ease my mind about my friend.  Does this stuff work out or should I be prepping for the fallout here?  Do cheaters change? He wants to marry this girl and has mentioned a few times that when he does I'm going to be his best man - kinda awkward.
<bot>: My best friend is in a relationship that started as an affair.  I can't help but be a little weirded out by it and don't know if I'm overreacting or what to think.