Things happen because the laws of nature say they will — because they are the consequences of the state of the universe and the path of its evolution. Life on Earth doesn’t arise in fulfillment of a grand scheme, but rather as a byproduct of the increase of entropy in an environment very far from equilibrium. Our impressive brains don’t develop because life is guided toward greater levels of complexity and intelligence, but from the mechanical interactions between genes, organisms, and their surroundings.
None of which is to say that life is devoid of purpose and meaning. Only that these are things we create, not things we discover out there in the fundamental architecture of the world. The world keeps happening, in accordance with its rules; it’s up to us to make sense of it and give it value.
I’m a fan of Sean Carroll’s contribution.
Excerpt:
Edit to add: Sean’s blog post on the subject, entitled Dysteleological Physicalism.