The answer to that question is, "because God put "South" in the bird." South was programmed in the bird at it's birth. To discover your purpose, start by asking yourself the question, "What's the thing that keeps pulling me "South?" What's the desire, that keeps coming up within me that's hard to ignore? This might be your purpose. Often we frustrate ourselves by looking elsewhere for our purpose, when the biggest clue is within us!
Stop ignoring the urge to fly South. Explore it. You will be surprised at much how fun flying South really can be.