Answer this question like you're a philosopher: Why do I have to eat my vegetables?
When we are children, we are mostly concerned with the immediate good: what can be of benefit to us within the next thirty minutes or so. Part of this comes from our lack of object permanence or finality, but part of this is because we are selfish creatures at heart. I say, however, that it is not for us to become unselfish, but rather to become selfish in a more fundamental way. Do not do the thing that causes you immediate pleasure if your future self may suffer the consequences. Do the truly selfish thing and plan only for your future. Eat your vegetables selfishly, for they will cause you to grow and mature into a healthier version of yourself. That is the truly selfish thing. 