Why do men need or pull away after sex?

I'm not really the close type. I just have sex and hope the guy will want to continue having sex with me as well as a friendship. But recently, I had sex and this guy pulled away like immediately. I called him once yesterday and today so I said I'm not calling anymore b/c I don't call people period. So I'm moving on and maybe another guy will come into the picture and replace him. But why do guys pull away after sex? You could be cuddling, kissing, etc and he's giving you the vibe that you two are connected but then he pulls away the next day. I read something about guys wanting independence or something?
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