Stress
Because it can actually get a bit stressful on a film set (you're working twelve-hour days on a strict deadline, you're on your feet the entire time, and each failed take is enough to put everyone involved in a rotten mood), sound mixers tend to be fairly stress-free individuals. At least, that's the way it appears on the surface.
It's sort of a "survival of the fittest" type of deal. Those who can't hack it have probably gotten out long ago, and are now repairing or selling home entertainment systems at Best Buy. Those who have stuck it out are pretty tough. Be prepared not to let things get to you. Learn to let things roll off your back, like the iced chai tea that actor just poured over your head because you were in her way.