I believe the most consistantly compelling motivation, the most determining factor in the human experience, is the fear of death. The core, most primary thing we know about being alive is that somewhere along the line we won't be anymore. Everything we are, do, think and feel is necessarily founded upon and grounded within the terrible, brutal fact that we -- or at least the we as we are now -- are as temporary as temporary gets.
At some level, this abiding (if unconscious) surety of the tenuousness with which we own life instills a panic in us all. It makes us nervous, greedy, appetitive, angry, fearful.
Mostly, the underlying fear of imminent obliteration tends to trigger the Two Great Human Drives: the drive to sex (whether it be physical gratification or the desire to reproduce), and the drive to power.
Sex and power. That's what makes our world go round. Those two things make all people -- Christians, Muslims, atheists, basketweavers in Mongolia -- absolutely bonkers. We want them. We crave them. We're repelled by them. People are so driven one way or another by them that it ... well, that it's created the world around us, for one.
But I don't think there are only two ways people primarily react to their innate fear of death. I think psychologists, theologians, and philosophers should add to the drives towards sex and power the drive toward constancy. I think the desire for constancy -- for permanence, changelessness, enduring equilibrium, unending stasis -- is every bit as compelling, as motivating, as crazy-making, as informing a drive in people as is their desire for sexual gratification and power.
Everyone, all the time, imagines themselves in an absolute state of peace and contentment. When I'm out on my own. When I get that job. When I land that deal. When I'm famous, meet the right person, get married, have children, retire, get to heaven. The need to achieve a state of indissoluble contentment is forever before us, forever pulling us ahead, forever pushing us from behind. It's what makes our personal world go round.
I think our desire to finally achieve a state of immutable harmonious equilibrium is as compelling as is our drive to sex and our will to power.
I also think it's one of the key reasons Christianity is so perfect. Christianity acknowledges that we can't live in a state of absolute peace while here on earth (while at the same time, awesomely enough, providing us a means to get back to a state of perfect and absolute grace before, just by being alive, we begin blowing it again). Moreover, Christianity promises us that when this life is over, we finally really will get what for our whole lives we've longed for, which is is peace and joy unending.