Atheism makes me happier for the simple reason that it means that I am responsible for my own choices in life.
I find the traditional Judeo-Christian god to be very paternalistic (hardly an original thought, I know). We can be good children and obey, or we can be bad children and rebel. We have no other choices, and we will never grow up—to god, we will always be children. Some people have told me that they find this to be comforting. It makes me feel trapped. We must live our entire lives following a set of rules imposed from above, and can never be free to decide for ourselves. I am happier in a universe in which we are free, one in which we can be the adults.
I feel more or less the same way about any other belief that there is a purpose to the universe. Whether the purpose is imposed externally, or created retroactively from the hypothetical Omega Point, it constrains our actions to acceptance or rebellion. I am happier if we are unconstrained.
I'm not trying to argue that the universe which I am describing, the one I believe to be the actual universe, is somehow better than some other universe, such as the one in which the Judeo-Christian god exists, the one which many other people believe to be the actual universe. I'm arguing merely that I am happier in the universe with no god, and, since there is no logical reason to believe otherwise, I choose to believe that there is no god.
Using the definition of religion I gave earlier, it should be clear that I am saying that atheism is my religion. I choose it as a matter of faith, not logic. I know that some atheists describe atheism as the logical choice, but I've never found their logic to be any more convincing than the various proofs of god's existence. I sometimes describe myself as a religious atheist. Obviously I'm stretching the usual meaning of the word, but at this point I hope it is clear where I am coming from.
I admittedly can't explain why there is something rather than nothing, but belief in god doesn't help much there either. You still have to explain why god exists rather than nothing. I've never heard of a reasonable way to answer the question.