Perpetual Theory of Life A purpose of life is to be successful in the domains (ecologies) you find yourself in or have voluntarily chosen for yourself. This success is about being rewarded and rewarding others in this game we call "The Philosophy Forum" which is a subdomain of the game of life.
There are as likely as many purposes (or justifications for being) as there are ways of being.The tinkering space at the end of the chain of being is our sandbox. We can do what we want within that space toward whatever goals we voluntarily or involuntarily accept as worth pursuing. A lot of that work isn't really our own, we can inherit purposes (functions) and abandon them.
I like the idea of universal Darwinism as applied to cultural artifacts (memetic theory) but you don't have to perpetuate it. Take it, leave it, turn in into a cartoon, or whatever, talk about Daesin, Genetic Imprinting, Jungian Typology, Jonathan Haidt's Moral Flavors, Economic Inequality, Black Box Metaphors, Biophilia et cetera. Obviously you'll always have reasons to prefer one kind of being (one direction) over another but you may not need really need them. No one has good reasons for eating at the Cheesecake Factory (oh but they do).
I can go outside and clone colorful variants of Cordlyine fruticosa and Hibiscus rosa-sinensis. Why do I do it? Because circumstances have made it easy, because they are beautiful, for a similar reason people put pictures on their walls (they see something as they walk by), to have something to talk about with other people who are interested in cloning variants, because I'm avoiding doing something I ought to be doing like long term planning, et cetera.