A natural consequence of being born into a world of limited resources, the compulsion to "pay the rent" is a drive that causes most humans to devote a majority of their productive energies to working at jobs
. Usually, this means doing something you would rather not do, in exchange for money that allows you to continue to live, so you can continue to work a job, doing what you don't want to do.
Biologists might say that we continue this cycle because of a genetic program that drives us to preserve our lives regardless of suffering, so that we can reproduce and nurture our children, thereby perpetuating the species. This results from the simple fact that those species that did not develop a self preservation instict have been wiped out, leaving only those of us who are stupid and stubborn enough to continue to struggle in a hostile environment. This process is known as evolution.
Buddhists have developed a philosophy, partially inherited from the Hindus, that recognizes the fact that life is suffering, and so they seek methods to break the cycle of suffering to "pay the rent," as described above, and to thereby exterminate all life. This may be viewed as a form of supreme enlightenment, or total genocide, depending on your perspective.
Of course, then there are the intellectuals, the artists, and the idealists, who generally tend to believe that the human capacity to love, to feel, to think, to create somehow justify our suffering, or at least partially compensate for it. This could be true, if in fact we have developed our minds to such a level that the life of the mind preached by ancient Greek philosophers is truly a life worth living. This, and the utilitarian view that strives for a world of comfort in which pleasure may some day exceed pain, are two leading causes for the development of the notion of morality. There may be some validity to these ideas, or they may be simply another evolutionary trick, filling our minds with absurd notions that force us to continue to struggle and strive in the face of little hope.