I have always interpreted the will to power in terms of evolutionary psychology. While that might not always be exactly what Nietzsche meant, for me it makes sense. Animals are driven to survive and reproduce and this means having or gaining control over resources. If you look at our chimpanzee cousins, within groups there is intense rivalry to gain status, because this ensures better access to resources. With humans, the survival pressures are now much smaller and our brain allows us to do more things, so this will to power that was originally a way to ensure survival is channeled in other ways.