Life is only validated by its own absence, without death life would hold no value. Life is precious to those who have lost it; while those who have not fail to appreciate why life is a game that must be played carefully.
Both the blessing and curse of life is that it ends. Without this end, we would not know to appreciate time as it passes. But the end of life, our own or another's, is cause for great pain, where solace may only be found by remembering the positive contributions of those we have lost.
So life in the end, once every breath is consumed, and all strength departs our limbs is no longer found in the continuation of our persistent understanding; but instead is found in that which we leave behind, our lasting impact upon society and those whom we have loved.
But why does this matter? Who cares? Why does it matter if we had a lasting positive impact? The answer is simple: because your interactions and impact regarding the world around you defines you. If you lived for your family, your family and the love that surrounded you defines you. But if you lived for yourself; alone, often drunk or high, always just looking for a good time, that selfishness defines your existence.
Love is classically the most positive thing in life. Its why people obsess over romance. But society's modern idea of love is severely shallow compared to traditions of the past. Society infers that love is a two way street, a give and a take.
In a romantic sense this is characterized as two individuals working for each other's better and happiness. But this is an expectant relationship, which demands some kind of service from your significant other. This understanding of love reduces the most positive aspect of life to nothing more than a contract which may be broken at any time should one party become dissatisfied with the arrangement.
Fortunately this understanding of love is wrong in all contexts. Love is more than a contract; love is a choice and an action. Love is commitment, in any kind of relationship, platonic or otherwise, love is a commitment to care for the best interest of somebody other than yourself.
Picture for a moment if people applied this understanding of love to interactions in everyday life. What would it look like? Wouldn't we be slower to become angry with each other? And swift to help each other? Wouldn't life for all of humanity become easier and better if we approached every interaction with the intention of love?
So, the purpose of life I think is actually rather simple: Love by choice. Impact the world around you and build your legacy in the most positive manner possible.