I’m quite sure they don’t since there are only several doctrines that unite almost all atheists:
No God.
No soul
No afterlife
First, these are not “doctrines” - atheism has no “doctrines”. Second, that is what atheism is all about: “the lack of belief in anything called supernatural”. But that is all. Being an atheist has no predictive value on one’s general disposition, on being kind or cruel, on being helpful or selfish on being cheerful or depressed, smart or stupid. No predictive value whatsoever!
How can you live a cheerful life under the assumption there is no God.
No problem there. The God you describe is not someone we consider “loving and caring” but someone who is indifferent - at best; if not outright malevolent and cruel.
I maintain that at least Sartre and Camus were honest enough to admit that an absurd existence produces not fun, but ultimately nausea.
Whatever you “maintain” has absolutely no significance.
The stats are in. People in religious countries are far less suicidal than in countries dominated by unmitigated secularism. How does that prove the cheerfulness of atheism?
You commit the old fallacy: “post hoc ergo propter hoc”. Even if it were true that atheists are generally “depressed”, which is simply incorrect, you would still confuse “correlation” with “causation”.
Whistling in the dark is a well known form of rationalizing our worst fears.
There is no fear of “afterlife” for atheists. For believers it might be a fearsome prospect to be thrown into the “eternal fire”. As the scripture says: “it is fearful to fall into the hands of the (kind and loving) living God (who loves you more than you will ever know)”. You are the ones who go periodically to confession, because you are scared of hell, of eternal damnation. One little unconfessed and unrepented mortal sin, and off to hell you go. Such a belief would lead logically to chronic depression, if it were “real”. But it is not. You all believe that you (personally) will get to heaven. Hell is for others. Now that is what I call “whistling in the dark”.
And I would like to point you to the moral theology forum, where you see the fear, the depression, the guilt of those poor people (mostly teenagers) who give in to the natural desire of alleviating their red-hot libido breathing down on their neck and perform a not just neutral but beneficial act of a little masturbation. They are the ones who are depressed and it is not a correlation but direct causation of their “theism”.
You see Charlie, it would do you a world of good if you actually studied the subject which you wish to argue. There is another old saying: “it is bad enough to be thought to be a fool, but it is infinitely worse to open your mouth and confirm it”. And I say this with benevolence, and being helpful toward you. (Of course there are some nincompoops who will consider it an “ad hominem”)