There doesn't need to be a reason for its existence. It simple is. Why do you exist? I understand how you came to exist, but can't think of why your existence is necessary. I exist, but I cannot, for the life of me, think of any reason why I exist. I'm the result of a fertilized egg due to my parents' sexual activity around Halloween of 1944. I know how I came to exist, but there is no why. I simply am.

The universe exists, as we can all experience through observation. It has some rules that exist due to its nature. We can't change those. It exists. Why it exists is of no importance, really. It simply does. Without its existence, we do not exist. We are part of that universe, albeit a completely insignificant part. We exist because it exists and we're a natural expression of its existence. That much is clear.

We can't really ask "why" successfully, because we cannot observe anything beyond that universe. Not a single thing. We cannot go outside of it to examine it, either. Since we are part of it, we cannot observe all of it.

We might be able to come up with the question of why, but it is a question without meaning, really, and certainly without an answer.