More than once (once most recently) I have been accused of not dating someone because I didn't think they were in my league, which was not the case. A lot of guys seem to think that if a girl rejects them it's because she thinks he isn't attractive enough, which is almost never the case. So here are some reasons I have rejected a guy.
1) We Didn't Have Enough In Common
We just didn't have much to talk about. Not many of the same interests and very different careers paths, it wasn't meant to be.
2) We Had Opposing Opinions
One or two differentiating opinions if fine, but if we disagree on every major topic then it's something difficult to work past. If we share different religious beliefs, different political views, and different views on equality, then we won't have much common ground. It's not that I don't respect your beliefs, it's just that I'd feel closer to someone who has the same beliefs I have.
3) My Family/Friends Didn't Like You As Much As I Did
Even if I find you incredibly hot and I like you, if my loved ones have a bad feeling about you, I'll probably cut things off. There's probably a good reason they feel that way and I trust them. It might not be because you are a bad person, but simply that they don't think you are a good match for me.
4) I Didn't Have Time For A Relationship
People use this as an excuse more often than not, but lots of us have genuinely been in a position where we just didn't have time. It had nothing to do with you, it was just that I was busy caught up in my own life to concern myself with someone else's.
5) I Didn't Think You Would Be Good For Me
You may well be a good guy, but if I feel that you would be unhealthy for my lifestyle or have any negative effects on me, I'll probably cut things off. Again, not because you are a bad person, but because you just lead a different life from my own.
So there you go. Guys, when a girl politely suggests cutting ties, it's most likely not to do with your looks or that she doesn't like you as a person, so don't always jump to those conclusions.
Girls, please add your own reasons why you have cut ties with someone you could have been in a relationship with! (Guys too, if you're up for it)