Picture it: the train is at the station. You've just gotten on and are standing there, vainly trying to grab hold of one of the poles spaced throughout the train. The doors somehow manage to close, and suddenly, the train jerks into motion. Your hand grabs for the pole, misses, and you feel like you're going to fall.You don't.
The belly of the fat, smelly man standing behind you bolsters you from behind. The lady in her business suit holds you up on your left. You're supported on your right by the guy bobbing his head to the tune on his iPod. Minutes later, the train stops. You still haven't managed to grab hold of the pole, so you begin to fall onto the lap of the micro-mini-skirt-wearing teenager sitting in front of you.
It's hopeless to try to grab that pole now. Better to just stand there and let the massive tide of humanity keep you standing in place.
NOTE: While I was looking for a picture to go with this post, I came across this article and this video. They're pretty good.