Boys and girls, that's a tautology. Of course we observe a universe with life – here we are! The question is, How could we discern between a designed universe and a chance universe? I think there are some ways, but theists don't seem to care much for them.
It doesn't seem obvious to me that the universe was fine-tuned for life simply because life happens to exist. The universe is for the most part an unfathomably vast frigid, lifeless vacuum. Over billions of years matter from the Big Bang clumped together, eventually forming stars. Those stars burned for billions of years. Some just died. Others collapsed in on themselves, forming black holes. Still others exploded in supernovae, seeding small parts of space with heavy elements. Over billions of years, remnant elements formed new stars, planets and solar systems. Most of them are far too cold, too hot, or too volatile to host life as we know it – save perhaps for some extremophile bacteria.
|A fine-tuned death for anything nearby|
So here we are on a knife's edge of survival, using human ingenuity to stave off the indifferent cruelty of nature. It wasn't any god that taught humanity how to survive – we figured it out over two hundred thousand years of trial and error. We're now poisoning the planet at a prodigious and unsustainable rate; if we don't annihilate ourselves through nuclear conflict, we still run the risk of destroying the only cosmic home we have.
Somewhere in the universe, more stars are exploding; more stuff is being sucked into massive black holes; more gas giants are being formed, as are rocks either too close or too far from their suns. All this happens has the universes' expansion accelerates toward a slow and unremarkable death as suns die and planets go barren.
All this is made worse by the simple fact that had God created the universe, there's no reason why he'd have to fine-tune it at all! We could be designed to life in the vast icy voids of deep space, or to comfortably live in gas giants, or to lounge idly on the event horizons of black holes. God could design us to live anywhere, in any condition at all.
Is this really a universe designed with life in mind? The far more parsimonious explanation is simply the inverse: We adapted to the universe. We were designed by the blind watchmaker of evolution to survive as best we can against the innumerable forces eager to kill us or just make our lives really miserable. If the universe were designed for life by God, he did a pretty horrible job.