Why does he act so weird after hooking up?

For the past 8 months or so, me and a close guy friend of mine have been in a vicious cycle. We have sex, then he acts weird around me for like a week or two. Things go back to normal, then we hook up again and the cycle continues. I know there's no other girl involved, especially since we keep hooking up. We've never talked about what we're doing and I've never tried to treat him like my boyfriend. Is it regret? Does he not enjoy it? I'm fine with our casual situation so I don't know how to bring it up to him, I don't know what's happening when we hook up that makes him act weird. He initiates it every time and when we get drunk he's all over me, then the next day it's weird again. He's definitely not new to casual sex, any ideas how to handle this or what's going on?
Why does he act so weird after hooking up?
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