well there isn't only purpose of life, the one we have defined for ourselves. There is much more purpose to our life than we can imagine. We don't know but we have been fulfilling this purpose since the moment we were born. Obviously achieving our self defined purpose gives us a feeling of completeness and achievement, but if we fail to achieve it then that does't mean that we are useless or our life is meaningless. We are unknowingly fulfilling our purpose of life.