How to deal with a friend who keeps AMOGing me?

AMOGing = Alpha Male of The Group. Example - Proving your alpha male characteristics in a group or trying to gain dominance over a situation. My friend keeps trying to compete with me and outdo everything I say or do. It's getting very annoying because I'm not someone who rises to the challenge and prefer to just be left alone in peace and quiet but he keeps picking on me trying to make me look stupid and small in front of my girlfriend and friends. I'm not the alpha male type so I guess it's easy for him to look tough but why does he keep doing this to me and how do I stop him?

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