My managers gets mad at me the most and I dont know why

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At first it did bother me and I notice that he gets angry at me directly whether it be for trivial things like slouching. He got angry at me and called me in front of all people then he just recently got mad again for trivial things at me but he did not scold the person I was with. I notice he never even scolds people directly calling their name but he does it with me. I really find it unfair but I'm getting used to it. I don't understand why he gets mad at me the most cause when he is in the good mood, we are the 'closest' friends and we often laugh around.