People believe in God for the very reason that they refuse to accept the fact that when they die, it will be almost as if they never lived at all. God gives people purpose and convenient answers to a very confusing world. That's why religous people are generally fairly happy. However, the majority of people that are athiests are so because they've been taught their whole life that they will not be able to make a difference and have come to accept that.

As for the notion that the universe is to complex to happen by chance, you are viewing complexity through the narrow window of human perception. Our universe in reality could be very simple. It could have been that the odds were stacked for the universe happening, and that the absence of the universe would have been far more improbable. Persoally, I lean towards the agnostic side that we have and never will have the ability to comprehend the nature of our universe, but what we cannot deny is that for whatever reason, we are here so we might as well make the best of it.