I remember reading about a thought experiment on altruism in my Philosophy 101 class. People were assuming that good deeds were done solely because of an expectation of reward, even if that reward was just feeling good about yourself. The question was, if someone was forced to take a pill that made them forget what they had done, would they still make a sacrifice to do good?
In the end, the answer was assumed that, yes, even if a person knew that they would be forced to forget the feeling of elation that comes from doing the right thing, they would still do it. Many ask the same sort of question of those who believe in heaven. Are their positive actions merely a consequence of expecting eternal rewards? I think that again, in this case, the answer is probably, more often than not, more complex than just a belief in return on investment. Richard Beck tackles this in his piece Heaven and the Arena of Spiritual Formation.
By contrast, if you possess a metaphysics of hope, where acts of love do matter, in an eternal sense, learning to love becomes a richer possibility. Our actions assume existential weight. This weight helps form love which will, when cultivated, begin to assume its own place and role in my motivational repertoire, and becoming, for the the saints, my sole motivation. As the Scripture says, three things remain. Faith, hope and love. But the greatest of these is love.
One may start a life of faith with a sense of reward or punishment directing their actions, but as that faith matures, the desire to do good out of gratitude, or simply for its own sake, becomes stronger.