The world consists of things, which obey rules. If you keep asking "why" questions about what happens in the universe, you ultimately reach the answer "because of the state of the universe and the laws of nature."
This isn't an obvious way for people to think. Looking at the universe through our anthropocentric eyes, we can't help but view things in terms of causes, purposes, and natural ways of being. In ancient Greece, Plato and Aristotle saw the world teleologically — rain falls because water wants to be lower than air, animals (and slaves) are naturally subservient to human citizens.
From the start, there were skeptics. Democritus and Lucretius were early naturalists, who urged us to think in terms of matter obeying rules rather than chasing final causes and serving underlying purposes. But it wasn't until our understanding of physics was advanced by thinkers such as Avicenna, Galileo, and Newton that it became reasonable to conceive of the universe evolving under its own power, free of guidance and support from anything beyond itself.
Theologians sometimes invoke "sustaining the world" as a function of God. But we know better; the world doesn't need to be sustained, it can simply be. Pierre-Simon Laplace articulated the very specific kind of rule that the world obeys: if we specify the complete state of the universe (or any isolated part of it) at some particular instant, the laws of physics tell us what its state will be at the very next moment. Applying those laws again, we can figure out what it will be a moment later. And so on, until (in principle, obviously) we can build up a complete history of the universe. This is not a universe that is advancing toward a goal; it is one that is caught in the iron grip of an unbreakable pattern.
This view of the processes at the heart of the physical world has important consequences for how we come to terms with the social world. Human beings like to insist that there are reasons why things happen. The death of a child, the crash of an airplane, or a random shooting must be explained in terms of the workings of a hidden plan. When Pat Robertson suggested that Hurricane Katrina was caused in part by God's anger at America's failing morals, he was attempting to provide an explanatory context for a seemingly inexplicable event.
Nature teaches us otherwise. 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.