God is a story to explain that which we experience but do not understand. It has been told through all recorded history and has become part of clan culture. The things we do not understand (until we research) range from physical phenomenom such as lightning through to biochemical such as elation and love. Since some knowledge is violently uncomfortable and difficult to research (what happens when we die?) there is ample motivation for stories to continue and multiply.
In a more reductionist fashion we can also describe God as 'that which is more powerful than us.' We have historically elevated people to that status.