Why do we argue so much when we're just friends with benefits?

he was my friend with benefits since 2009 till 2011 we've always been off and on with our friendship because he always does something to piss me off or that he knows is wrong throughout the years I have tried to pursue a relationship with him but every time he refused and just wanted to continue to be friends which was fine with me so not only are we not in a relationship bt he also has made it clear to me that he does not want to be in a relationship with me I feel like I'm going through the motions and we're not even together one minute were cool & hanging out but the next he's making me upset about something then it's an argument ugh why we can't get along for two seconds when your not my man?
Why do we argue so much when we're just friends with benefits?
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