In short because things move from a state of higher energy to lower energy. Not subduing yourself to gravity would take energy in opposite direction to counteract it (or the geometric slope in this analogy). Newtonian conservation of energy is a pretty good model analogy (although isn't the same thing) and the geometric one is even better. But that's all they are, however accurate - analogies, constructed on the basis of available information and mathematical models.

Outside of that there really isn't a 'reason'. This isn't how physics works, although it's certainly in our nature to create and expect meaning and cause everywhere. Brain is bound to think in hierarchies, because that's how it's built to operate, but the universe doesn't have that limitation.

Good discussion of your question: http://physics.stackexchange.com/questions/24359/why-do-obje...