When I was 16 years old (I am now 23) I was 5'6 and 220lbs, I dropped the weight by doing heavy cardio and light lifting to 150lbs in 6 months. I was sick of getting picked on, ignored, and looked down on because of the way I looked. So I decided to do something about it, lost a ton of weight, and then started dating one of the hottest girls at my school =)
SO anyway, ya, its a "IN YOUR FACE" kind of motivation for me. And no matter how much time passes, that little fat kid is still inside me, and the "HA! I CAN DO ANYTHING!" motivation is what keeps me going today.
I also find myself focusing more and more on my fitness/running when something bad is going on in my life, when I start to feel a wave of depression (hereditary) come over me, I know that in order to counter it, I need to start to pick up my workout. Give my mind something else to focus and strive for other than my own depression and thoughts. My parents even notice it, the more I work and focus on fitness, the calmer and happier I become.