Why does my boyfriend gets jealous when he cheats on me?

My boyfriend cheats on me with other girls. There are times where he will ignore me all day and talk to his girl "friends" in private and slam the door in my face when he with them. I know he is flirting with them and trying to hook up with them. He hasn't been able to hook up with one yet but he still tries.

With me, he will cock block any guy that talks to me. I have this guy friend who is gay but you wouldn't know that unless you asked about his sexuality. When we go out, my boyfriend has to come along with us. He will follow us and keep a sharp eye on him.

I did the same thing to him when he's with his girl friends and he pushed me away.

He only tells his friends about me unless he thinks they are unattractive or if they have girlfriends of their own. He doesn't hang with any of his attractive friends around me.

There was this one hot friend he hung out with when we went to this party. His friend randomly walked up to me and called me beautiful. I smiled at him and said thank you. We were smiling at each other and my boyfriend looked annoyed. After that, my boyfriend told his friend I was his girlfriend. His friend got upset then told him he was a lucky guy.
This was funny because that rest of the night my boyfriend kept a sharp eye on me the rest of the time at the party. When his hot friend left him to go somewhere else, my boyfriend would go looking for me to make sure I wasn't with him.

Now every time we go to parties or in public, he watches me the whole time.

Oh yea, he doesn't hang around that friend around me either.
Why does my boyfriend gets jealous when he cheats on me?
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