I started because I was consistently above 190 lbs and gaining about a pound every few weeks. My younger brother was loosing weight and provided me the motivation to do it as well (he can't be thinner than me!)
I have found that if I limit my calories to ~1,500 per day with ~2,300 calories burned, I can loose weight aggressively. At ~1,800 calories per day with the same burn I am loosing about a pound every week or two. I'll stop at that pace in June when I go on vacation.