The single most important principle I ever discovered is this: the goal or purpose of the Christian is precisely the pursuit of happiness - in God. The reason for this is that there is no greater way to glorify God than to find in Him the happiness that my soul so desperately craves.
Curiously, a principle affects your life whether you are aware of it or not. For instance, the principle of gravity was working long before the apple ever fell on Newton's head. But once it did, and he understood it, then we as a society were free to harness this principle to create, among other things, airline flight.