Why does my boyfriend hate my friend?

My boyfriend can't stand one of my male friends for no particular reason. He isn't the jealous type and he likes many of my male friends. He never admits not liking him, but I see he hates when I hang out with him alone. This friend has liked me in the past and my boyfriend knows it, but he likes my male friends who have also liked me and doesn't mind me hanging out with them.
Why do you think that is?
Why does my boyfriend hate my friend?
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