We use our ability to reason for a great deal of things, and I think the answer to this sort of question lies mostly within the definitions used. What is reason, and for that matter, ultimately, what is truth? Can we reasonably determine the cause of the universe? Or even ourselves? Let's take a look at some of these questions and see how things shake out.
Why is it more logical to say that God has always existed? Surely if God could always exist without cause, the universe could too. The truth is, at face value there is no logical difference. If something can exist without a beginning then anything could exist without a beginning. Why should it only be one thing?
What was there before there was a universe? Before the Big Bang? From the moment we realized our universe had a beginning, we have been wondering what caused it. Could there have just been nothing? Can something come from nothing?