No particular reason I guess? Life is its own purpose of existence, it's just a varied system of complexity. My understanding of the universe is that of "lumpable" systems (Blackboxes as you will) that build on themselves without exactly needing to understand the inner workings to do so. Bacteria build on chemical compounds, which build on atoms which build on etc.That also means that when you're inside the Blackbox you will work fine without knowing what system underly you and what's above you can only be guessed at. So at best, life is meaningless and that's what's nice about it. You can't really mess it up except for what you define as a mess. At "worst" we don't know, and maybe we will never know but at least we'll have this question to ponder.