There can be a million reasons why someone is motivated to train. Some peoples motivations are broad or clinically precise. Whether it is to have a physique similar to that of a role model (personally The Rock has what I call an ideal physique), completing a certain event, wanting to catch the eye of everyone at the beach or to be fit enough to keep up with your kids.
For myself, my motivation comes from competing. I achieve the best results with my diet and training when I am training for something in particular (I.e football and kickboxing have been my past sports). This is also an issue because when I don't have a sport to train for - Everything goes out the window, which pisses me off. This is my scenario recently, but having decided that I will get back into playing rugby next season, this gives me something to work towards.
The body is an amazing machine and it's surprising what a lil' motivation does to help us get to our goals