As an undergraduate studying philosophy in the 1970s I spent much time reading atheistic philosophers. Some of these readings were required for classes, and others I read on my own.
I was introduced to Nietzsche and Sartre and Camus, to Flew and Humean skepticism (whether or not Hume was an atheist is questionable). I read Kafka and watched the films of Ingmar Bergman. I immersed myself in atheistic and theistic existentialist literature, finding it fascinating and challenging and compelling. I also found it, personally, despairing. And I respected the reasoning and thinking as regards the logic of atheism which, for me, still means this: If there is no God, then this life is meaningless and absurd. One can coherently imply nihilism from atheism.
Consider Bertrand Russell's famous quote from "A Free Man's Worship": "That Man is the product of causes which had not prevision of the end they were achieving; are but the outcome of accidental collocation of atoms; that no fire, no heroism, no intensity of thought and feeling, can preserve an individual life beyond the grave; that all the labours of the age, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction in the vast death of the solar system, and that the whole temple of Man's achievement must inevitably be buried beneath the debris of a universe in ruins -- all these things, if not quite beyond dispute, are yet so nearly certain, that no philosophy which rejects them can hope to stand. Only within the scaffolding of these truths, only on the firm foundation of unyielding despair, can the soul's habitation henceforth be safely built."
I felt years ago and still feel today that, if the truth is that there is no God, then Russell is correct about this. One seems left with the oxymoronish truth that our lives stand on a "firm foundation of unyielding despair."
I remember reading Camus' "myth of Sisyphus" and thinking, "but of course if God does not exist then this is my life." I read Kafka's "Metamorphosis," even picking it up in German (Die Verwandtlung), and thinking that here was yet another expression of the logic of atheism as Gregor Samsa wakes one morning to find he is a beetle.
None of this is an argument against atheism. Rather, it is the logic of atheism as some see it and as I see it.
I have no doubt, indeed I am quite certain, that there are strong atheists who feel happy today, who are happier than I am. As for weak atheists, those who say they disbelieve but can give no compelling reasons for their disbelief, I view their lives as admixtures of proclaimed godlessness and inherited religious ideas; that is, atheism polluted by theism. But that an atheist feels happier and more alive than I ever have means nothing to me since I have never found belief and unbelief to be functions of one's moods. Does atheism lead to a life of inner emotional despair? Not necessarily. But if there is no God does our life rest on a foundation of unyielding despair? Of course.
