Partly, this banding together occurred out of necessity, to protect the human race from predators and to share the skills and wealth accumulated by individuals. This is just an anthropological glimpse at how humans were destined to be social though; our biology also seems to have played its part.
With a larger and better-developed brain than other primates, we were practically built to be social. Our evolved brains mean we can reason, speak, problem-solve, learn and interact socially.
It is this ability to learn, communicate and evolve that has contributed to our survival as a dominant species and allowed us to prosper as a civilization. Sociality is just as important to modern human life though, as psychologist Adam Waytz explains: "sociality is a dominant force that shapes thought, behaviour, physiology, and neural activity" (source).