I feel sorry to say this, but I think if a man says that, in his mind he's still waiting for "the one". He doesn't deserve to get all your heart and expect you to give all if he isn't willing to give all of his to you. :-(
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. :/