What would make a person not want a relationship?
Just enjoying the single life with no ties ?
There are many reasons but if you want to find out his ask him.
Hopefully he'll be honest.
(Though if part of it is that he doesn't/wouldnt want a relationship with you personally, don't expect him to be honest about that - because no-one wants to say that to someone's face).
I once asked a guy this - I was so persistent and so straight that he ended up answering, the answer being "I just didn't fall for you".
Felt emotionally like taking a bit in the stomach from Klitschko, but it was actually the best thing he could have said, both in regards to him, and to relationships in general. Sometimes people just don't fall for you, they like you and may be attracted to you etc etc but they don't get strong, definite "I want to pursue a relationship with her/him" feelings, and that is likely to not change.
Incidentally, with distance, I realised I don't even like that guy all that much. He's likeable in some ways, in others he's shallow, pretensious, faffy, judgemental, had issues, has a religious zealot mum who would be horribly interfering etc etc.