Is it normal to always initiate arguments?

Cases in point:
1. My sister said "Art students are night owls." She is an art student that just graduated college so yeah she probably noticed a trend and has credibility for this, but everyone knows that its a generalization-which can often be proven wrong. SO because I knew there was even the slightest possibility she was wrong, I argued about it. I could have just let it go, but I didn't.

2. I had an ex-boyfriend that I always argued with as well. He'd say something like "I'm playing a game, I'll call you back," and I'd respond like "Is it that more important?" even though clearly we all knew that he meant it with a grain of sand. I just wanted to fight.

3. One time, a good friend said "I don't like our teacher." Honestly, no one really liked her: she was way too political and he is gay so of course he was offended sometimes. BUT that did not stop me because I managed to point out her good qualities just to be on the opposite side of him. I didn't do it to make my teacher seem less evil, I did it so that I could argue with him.

ALL of these instances could have been avoided, but I don't avoid them. It seems like I argue just for the sake of being right sometimes. I was in debate so maybe that's a habit I picked up, but why can't I just be nice?

I am going to change.