Here's a great article and discussion on the ultimate question facing science (and religion, and philosophy): Why is there something rather than nothing? Doesn't the principle of entropy make nothing (or at least a dead, inactive universe) more "natural" and likely than this complex cosmos we live in?
One response to "Why?" is "Why not?" Another is that we have only one universe to examine, and that doesn't tell us anything about the possible alternatives. It's an endlessly fascinating (sometimes maddening) question.