Here is an interesting article about the ever-so-popular neurotransmitter, dopamine.
The article suggests that dopamine is not just a "pleasure" or "reward" neurotransmitter, but also the key to motivation. For example, rats with very low levels of dopamine will starve to death even in the presence of food... they just don't seem motivated to eat it. If you put it in their little ratty mouth they'll eat, but if they have to walk a bit, forget about it. They just starve to death.