This post is a short one, but I find there are lots of different reasons for different people. Some people work out to get healthier, some to get stronger and some to look better. I have friends who workout so that they can get faster in track and field. Personally, I want to get stronger and slimmer. I have a wicked sweet tooth and working out keeps me from ballooning :) Also, after a while I get into a groove; now I look forward to my bike rides and intervals. a really good system is to set short term goals. When setting goals make them specific and not based on the scale, because a scale doesn't represent how healthy you are, its just the force on you due to its gravity (thanks Wikipedia). Right now I'm working towards a belly-baring costume for Halloween.
So what motivates you?