The 3 reasons to do anything
One, to make a lot of money. Two, to change the world and three, because you can't help it.
The first one does not merit much discussion - you need to put food on the table, provide for the family and you do what it takes to maximize your marketable skills to make the most of it.
The second is different in that you have to decide in which way are you going to try to "change the world" or at least contribute to it. Do you help starving children or do you work to help the elderly. Do you try to make a difference in Darfur or do you try to make a difference in the inner city Detroit.
The easiest of them is the third one. I asked a friend who is a singer "if you were on a deserted island and there was nobody else around, would you still sing every day even if there is no fame or fortune to be gained?". People who have born talents just can't help but use them and of course she said yes.
Most of us in life ever get a chance to do one of the above and the very lucky get to do two or even all three. Those of us without natural talents can only lament that and try to do the One and Two ...