There are several answers I give whenever I do answer this question which one I give depends on what I feel like at the time, the truth is that they are all correct for me it was a comming together of many factors that made me want to train.
1. I used to do Kickboxing and I did circuit training (a lot of body weight exercises mainly) I did these for 3 years and was in decent shape as a result. I left University and stopped all this activity, a few years later whenever I talked to someone about weights and fitness I found myself saying "I used to...." a hell of a lot. I got pissed off at myself for living in the past and viewing how good i was by how good i was back when I did sports, I didn't want to say what I used to do I wanted to be able to talk about what I did now.
2. I am the eldest of 3 brothers, being older than one by 4 years and the other by 9 years. As they got older they got stronger, now as I was a fair bit older I was way stronger but as years passed they were getting close to me. I could not accept either of them becomming stronger than me so I started to train.
3. I did my Masters Degree in Sports Science - Fitness and Health, I hated telling people this whilst looking like crap, if I ever wanted to work in this field then I figured I needed to look the part so people would take me seriously. It'd be like a mechanic driving around in a car that always broke down and he could never fix it right.
4. Respect, I wanted respect, I wanted respect from people even if they only just glanced at me whilst passing. Very paralell with this I wanted people to be jealous of me, I wanted people to wish they were me. This sounds very wierd, but it's true, guess it's some psychological thing.
5. After I started Training MANY other factors played BIG parts on keeping me going.