Human ingenuity, indeed the whole of human history, has been focused solely on the achievement of two things. These are the furthering, and easing, of survival and once survival is reasonably assured, the attainment of happiness. At base, we are motivated by an abject fear of death, but because we consciously realize the inevitability of death, or rationalize it through religion or denial, we can focus on making the most of what is before us.
Thus, we are driven to find happiness, not only for ourselves but for those around us. This last point is one of the most heartening facts of psychology. We are social creatures, not only as a means for survival but also as a means for achieving greater meaning through group belonging. An isolated life, one that does not include a collective belonging, can drive a person either to new heights or to the depths of psychosis.
That is not to say that the only purpose in life is to exist as a part of a group. The purpose is rather to achieve brilliance within a group. You could, for instance, make a great medical discovery, become a millionaire, or achieve respect as a great lawmaker, two of these are achievements in society as a whole, but one is a personal achievement. Medical discoveries and respect among peers are both accomplishments that either benefit society or are a result of perceived benefits to society.
Money is not the root of evil, but a mere coincidental companion. The fact that ruthless people often gain the greatest wealth causes this association, but fear not! The social pressure often bestowed upon those of great wealth causes them to adopt the habits of extreme kindness, also known as philanthropy.
So in conclusion, yes, we are essentially selfish creatures, driven to comfort, however, our means of attaining it, namely belonging, coincide with the well being of society as a whole. The role that evolution plays in this is beyond the scope of this article, but it is quite possible that the survival of the species, or perhaps not survival itself, but the development of advanced societies capable of sustaining large populations depend on the investment of individuals in a group.
No theory can seek to explain the depth of human motive, but happiness seems to be a universal force behind every man, woman and child, even if it is in the context of society as a whole.