I think the main fundamental drive is not about getting a reward. It is about building a relationship with the Creator of the World. The reason for doing the mitzvos is that they are an expression of His Will, and we love Him very much, like one loves his father, and obey his will not because of the reward but because we want to do something nice for Him.
Let me elaborate on this. One idea is that there is a tremendous element of hakaras hatov which is the basis for everything else. One cannot notice the kindnesses of the Creator towards himself, and it is natural to try to want to give something back, even a slight little bit. Like with your parents, you can never pay back fully what they gave you, but giving back what can be possible is the least a human being can do.
The Creator does not need us to do anything for Him, but in His mercy, He created these mitzvos to give us an opportunity to give him something back.
Another idea, is that the reward is not what it's really about at all. The argument is put forth by R' Moshe Chaim Luzzato in Daas Tevunos - if the world was about the reward, the free will (main instrument for achieving this) should have never been terminated. But we know free will at some point in the future will be abolished and the Creator will reveal Himself fully, so for Him, the purpose is in revelation, not in reward.
A teacher of mine once pointed out that people believe life is like a video game - you collect some bonus points and some demerits, at the end of the level, the demerits get subtracted from the points and one (hopefully) gets to go to the next level. That's not what it is about. Life is a relationship. Like when you take the garbage out at home, it earns you favor with your wife, but the favor is not quantifiable. You take it out to make her feel good, to do something nice for her, not because she will cook you dinner afterwards. It is much the same with the Creator, who seeks a relationship with each and every human being.