We are only friends with benefits, why is he pissed at me?

Yeah of course he doesn't want a relationship with you. Rubbish. He's not admitting he does in case you don't want that and he ends up looking like a fool but he behaves like a jealous boyfriend because that's exactly what he is. Wakey-wakey. Friends with benefits includes sex I assume and that in turn creates emotional attachment whether you like it or not. Just ask him out and be done with it.