Not a take that tries to instruct or offer advice. Simply my observations from both myself and other guys i've talked with.
*Note: This does not apply to ALL guys. This is a trend that i've noticed. If it doesn't resonate with you then don't discard it.
1. He's horny
When a guy is horny and isn't in to being a player, he may often try really hard to get into a relationship with any girl chemistry be damned. When he's told no he doesn't really feel bad. He simply is like ok and moves on hoping that if he sprinkles a lot of seeds in the garden one will eventually have to bloom for him. Rejection to him a matter of arithmatic and only addition at that. Girls shouldn't feel bad for saying you don't feel a spark but SHOULD be upfront because he's not keen on sticking around to talk about movies or vacation spots or favorite music with you without sex happening sometime in the future.
2. He's on a love saga
Like Ted from HIMYM or Ross from Friends, he's actually trying to find that right woman and sees each one as a quest. If you end the quest, he's finished. He may not take the rejection as well, but you're helping him on his journey by letting him know where the dead ends are.
3. He's macho
Guys don't have straight female friends. To have female friends makes you a wuss. How can you stand to be friends with a girl who wanted to have other guys do her over you? This is the thought of many a macho male. He may honestly be fine with you and like hanging with you but machoness is machoness.
4. You were never interesting to him
Some guys have the gift of being super physical with any person and this helps tremendously in leading to sex. Other guys have the gift of being able to gab with any person but this often helps them to the friend zone. He can laugh with you, he can have deep conversations with you, he can cry with you but...he can do that with anyone. If sex is off the table he has to admit, you're kind of boring...
5. He's insecure
He has taken your rejection, one girl out of millions, as a sign of his value and as such he feels valueless when he's around you. This is the guy I most try to help on this site as it's a sad sight to have to witness. He has no personal purpose no personal sense of values and respect for himself that makes him feel worthy even if you don't agree. He doesn't understand that it isn't about agreeing or disagreeing since it's true you feel that way. It's about not caring. Since he doesn't understand that he's in a mental prison of emotional flagellation. Best to just let him suffer silently...