Why would a guy give me a hard time and sometimes target?

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Coworkers and myself noticed he will even pick on me, and it it’s mainly me who he does it with, ( been told that maybe he is just comfortable with me) and this man is relationship by the way! But also like one time I was in group laughing and he started making fun of my laugh situation personally thought it was rude and and just be (bipolar) like one day very nice next day rude or what not what’s his problem?
Why would a guy give me a hard time and sometimes target?
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