I was a bike messenger in NYC for about a week ca. 2000/01. I quit when I realized that, even in that short amount of time, I had already mostly stopped yelling at drivers -- making them actually hear me and recognize that I was there -- and was getting used to just adjusting my actions to compensate for their dangerous behavior. I thought, "I'm already getting complacent, and going to wind up paralyzed or dead like this". So I dropped it. It was fun while it lasted, but I hadn't yet figured out how to actually make it pay, so no great loss.