I think meaning can only be bestowed on us by the thing that put us here. If you craft a toy soldier it is not up to the soldier to decide it's purpose, but you as its creator to decide whether to give him away as a gift or put him on the shelf or whatever you deem worthy. I believe that just as the toy soldier is a product of the craftsman's hand, human beings are a product of nature. It is therefore nature that gets to decide its purpose for us and I think it has decided unequivocally that we our main purpose in the lifetime it gives us is to create more of ourselves. The arc of history, from the birth of the universe to the formation of the planets to the first spark of life has been an arc of creation. When considering Mother Theresa et al, I'm not arguing that you can't break out of this natural construct of purpose and effect great change in the context of our own constructed societies. What I do argue is that if there is just one thing that could be called the purpose or meaning of life, the one common thread imbued in all of us by nature, it is propagation. And I don't think it is coincidental that it is also the most fulfilling. Maybe Mother Theresa missed out.