Why is he acting like a caveman?

I don't understand men anymore.

I broke up with him a few days ago, because he NEVER wanted to be in a relationship with me (officially) for months.
But today he saw me talking to another guy (friend) and he came beside to put his arm around my waist. I told him to not do that but he just shrugged and left without saying a word.

Then he sent me a text message saying "I can't believe you flirt with other guys in front of me."


What's his problem exactly?
Why is he acting like a caveman?
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