Sure, you know in theory that every human being is a complicated piece of work, but in practice it's always easier to lump together those who belong to a group a) we don't understand, b) we don't like, c) we do like, or d) whose constituent members dress almost exactly the same.
2. They Don't Choose Where to Go
You can certainly state your preferences here, and occasionally you go to one of them, but mostly you don't have a clue as to which of the 400 or so missions in the world you'll be sent to. This explains why the opening of the big white envelope containing your mission call is a gigantic ritual, replete with your 300 closest friends come to hear you read out your exciting place aloud.
3. They Don't Choose Their Companions
Although you are always in pairs, it's not because you necessarily want to be (see #1). Your companion is assigned to you by your mission president. There's the possibility of a transfer every six weeks or so, in case you're not getting along or are engaging excessively in the favorite pastime of arguing over such high theological matters as whether this Mormon teen-celebrity or that should go on a mission. Pretty soon you learn that you're best off just trying to make things work with whoever you're with, which isn't a bad thing to learn for real life.