why does he hate me for no reason?

[Matt]

Because you had sex with him. He got all he wanted from you. On to someone else. Sorry sweetie but that's the way us guys roll.

It doesn't necessarily mean that he hates you, but he's done with you. You'll figure this out as you get older, but us guys don't really want you to give it up to us so easily. We want you to playfully reject our advances for awhile until the time is right. The girls who reject us the longest make the best girlfriends (maybe even wives), because we know that we can trust you, and you aren't likely to cheat on us. Just don't reject us for too long, or we'll start thinking you're not really interested. But when a girl we have taken interest in falls for our advances hook, line, and sinker very early on, we will take it, but it's one and done. Our logic being that if she gave it up so easily, she won't hesitate to give it up to another guy behind our backs.