Your purpose in life is not so much defined as it is implied. And where it is implied most, is in the example of young children, before the creation of identity becomes increasingly important.

A child is naturally dynamic. It is naturally fun-loving and interested in life. It is eager to learn. It wants to develop its physical strength and its knowledge of how things work. It revels in the use of its body and in sensual experience - how things feel to touch and to taste. It expresses emotion freely, preferring to return to a state of natural equilibrium (at- peace) very quickly, rather than to dwell on the emotion. It finds challenge wherever it goes and it applies itself to those challenges with determination and with courage. It seeks to enhance its life by pursuing opportunities to have fun and to enjoy what it does, naturally extending itself to create challenges.

And most of all, the child exhibits a natural intimate knowledge of the process of becoming happy...