I trained with a trainer last fall doing insane drop sets and to failure, being able to only limp out of the gym after it all. But when I had to travel and train alone, I realized how dependent I had become on his type of partner training. I prefer training alone because I'm not worrying about a sicko partner not letting me stop when I'm ready to drop or yelling at me to hustle to the next exercise or whatever. Kinda pushes that fine line between motivation & irritation. As far as a lifting buddy, I try to keep my set intervals to 30-45 sec. I get pissed off, but am nice about it, if someone asks to work in with me.
So, on go the headphones and off goes the smile -- time for pain!