One word - Discipline
As I wrote in my training thread my reason for training is to be the best Dad I can be, to be a dad that CAN. Can run with his kids, jump, climb, play, lift things, build things, make things. As my kids grow up I want to be able to keep up with them and set a good example. That in itself is motivating.
For me though its more about discipline than motivation, I am the sort of person who thrives on routine and loves a process. For me training is just part of the process of life, as much as eating and sleeping and work and family time and recovery are. Once I have said i am going to do it there is no need for motivation I just do it, I remain disciplined and treat it as something I have to do. For some this wouldn't be motivating but for me the more training becomes like work (like something I have to do) the more I thrive on it.