Well, I've been a guy my whole life and I guarantee you that if he's not interested it's for at least one of these reasons. You may or may not agree with them. Some of them are cold, insensitive and shallow; but whether you like them or not, here are the possibilities:
1. He's not attracted to you
Attractiveness is key to any romantic relationship. It just is. If he's not attracted to you he'll never admit it because he doesn't want to hurt your feelings. Sorry hun; sometimes that's just the way the cookie crumbles. You'll find the one.
2. He's afraid of commitment
Change can be scary.
3. Or he dislikes commitment
He may view commitment as a restriction to his freedom ... like pants.
4. He's afraid of the future
It either ends in marriage or pain ... or both. That's daunting.
5. He's afraid of the past
Bad experiences in the past tend to affect present decisions.
6. He's afraid of women
Literally. Gynophobia. It's a thing.
7. He has other priorities
Maybe he puts school, his career or Star Wars collectibles ahead of you.
8. He's weighing his options
Maybe he's dating multiple women and can't decide whom he likes more.
9. He's unsure how he feels
Maybe you are the only girl he's dating and he just can't decide if he likes you more than his ex-girlfriend's roommate.
10. He thinks you're too good for him
That's flattering, right?
11. He thinks he's too good for you
This is less flattering.
12. He just got out of a relationship
He may want to wait to make sure you're not the rebound.
He may also be worried about how swapping ladies so fast will affect his reputation (this is lame) or about the feelings of the last girl (this is less lame).