If they enjoy it, that's great. But, even if they don't, there are good reasons for them to learn and work. The good news about motivation is that it is not inborn. We can, and do, have a great influence here. Motivation is stimulated through modeling, communication of expectations, and direct instruction.
Motivation is about putting forth and sustaining effort in order to learn.
For example, I would love to be an astronaut. I put great value on the service of these people. But, for me, the expectation of success, is simply not there. I have about as much chance of making a successful shot to another planet as I have of being 21 again. Not on your life.