Guys! I need an explanation?

My best friend and I ended up being a "thing" for a while until he left me for his ex and used me as a random hook up and barely talked after. Time went on and we started talking every now and then I thought we were finally mending our friendship. So we decided to hang out, and ended up getting super drunk. He carried me into the bedroom and begged me to have sex with him and told me he still liked me, everything was going to be okay, and that he'd take care of me. I finally said yes and he hasn't talked to me since. We both lost our virginities that night, and we used to be each other's best friends.
Why would he do something like that? I mean, what goes on in guys heads?
Guys! I need an explanation?
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