I pick easy most of the time because I am more interested in the story than the gameplay, although I use the game play as a stress reliever and an immersifier (if that is a word). I only go on the harder difficulties if a: Story is lacking (ie fighting and racing games, these i normally go with normal/ highest difficulty) b: Achievements and i feel achievement whore-ish and c: Playing on high difficulties with friends is endlessly amusing. Perfect example is L4D2, with 3 friends on expert in the mall level. We were at the part where we just climbed over the dumpster and were heading into the building, all of us with low health (50 or lower).
Dan (who had the least: 30 temp health) goes into the building first and shouts TAAANNNKKK! He runs back to us, and finds Andy dead, and Jeremy and i barely alive, and says to run, cause he will take care of the tank. It Bursts through the door frame at which point dan, again, screams: Fuck you, Tank!!, and runs at it with a chainsaw. The tank hits him and kills him.