Father-in-law is constantly confrontational?

My father in law refuses to listen to what I ask of him. He always wants to be (always) confrontational with me. I've tired a few different approaches & nothing seems to work, with the exception of having to raise my voice. I'm not comfortable with doing this, because I know he can hear me & what's being asked of him. His son asks him to do things & he (completely) complies to whatever he says with no problem(s) what so ever. However, since I'm dealing with him, for 12 hours a day, I'm becoming a bit confused as to why he doesn't? To me, it almost seems as though he playing games with me. He seems to want me to get upset with him. I don't understand that "concept?" Why does the one that does the most for him, feeding, changing his clothes, showering, & shaving him does he do this with? I need some helpful suggestions please. I'm a bit, not only confused by this behaviour, but why he continues to do this for the one giving him the most amount of care?