I can't say for other people. For me it would be because I love someone else though. If I didn't love anyone else, it could either be that I wasn't sure about my feelings or that I just wasn't interested in a romantic relationship with the other person.
I have no idea. You should ask him. Also he probably has no idea of what is going through your mind unless you tell him.
I'm not sure. I guess he's thinking that the notion of soul mates is BS. :/