The through-string which binds us all is compassion. Think of it like this:
Without compassion, our species would have never survived the Savannah out there in Africa. Our bodies have not developed a special armor to repel attacks from wild animals such as the saber tooth tiger. If you can imagine what the African landscape contained way back when then it isn't difficult to surmise the development of the human mind to become this vast center of information processing. This includes defense mechanisms and any other survival traits - qualities which are imperative to the survival of the human species.
Our existance includes emotional and physical needs that are tended to by our will to survive. In our fragile physical state, we are susceptible to extinction, however the human mind has developed in a way to combat the evolutionary strife and survive in a vicious climate.
In order for our race to survive we must depend on each other as we have for the last 'x' thousands of years. Without this we are doomed to extinction. The simple key for our longevity is compassion. Again, without each other we can not survive as individuals, therefore every one must care for the other in order to survive. This has been written into a mulitude of dogmas all across the world. It is ingrained within all of us the moral basis to continue. For what purpose, one can only speculate. There are many theories from around the world as to what it means to strive toward existance.
Care for others as we care for ourselves - A simple rule that pertains to every religion, race and creed.
Plain and simple: We must all depend on each other to survive. There is no such thing as existance within individuality; Only as a group.