If there is no God, and this world is a merely-determined/ accidental and purposeless happening; then life is also purposeless.
If life has no external and shared meaning, then all that matters is how we feel about it - so life reduces to psychology.
Our innate psychology includes all kinds of instincts including loyalty, courage, appreciation of beauty, natural virtue... but with atheism these are accidental and contingent consequences of our past evolutionary history - and their meaning is in-the-past; and that meaning was only to enhance our differential genetic replicative success (because people are organisms, and organisms are only disposable contingent gene-controlled robots that exist to promote gene replication).
We may feel the pull or push of natural ethics or 'higher instincts' but they have no validity for 'my' actions, 'here-and-now'.
Therefore psychology reduces to the pleasure-suffering axis.
In other words; the only psychology that can be justified (for me, here, now) is my feelings, now = hedonism (the principle that the axis of my pleasure-suffering is the only 'ethic').
Current - here-and-now - feelings are the only relevant factor; because the future cannot be reliably predicted and the past may be forgotten or memories may be false.
Hedonic feelings are the only self-justifying feelings, because (by circular definition) feeling good is better than feeling bad. And because (with atheism) nothing else matters.
So with atheism hedonic feelings are the only thing left-standing.
And when short-term pleasure/ happiness, gratification is the only bottom line; then as soon as it is absent, life is worthless.
If we come to believe that our own personal happiness is in decline or impossible - or simply currently insufficient: too weak or too infrequent - then we shall (we realise) despair - sooner or later. And there can be no reassurance against this despair, because such despair is rational.
Under such circumstances, only the deluded manic or the manipulative psychopath can be consciously happy with life - and even they are vulnerable to downswings.
What happens then? Look around.
We see a society incrementally being overwhelmed by a tidal rise in despair; caused by atheism and the ethic of hedonism.
Amping up the pleasure is ineffectual - unless it could be made overwhelming and continuous (which is the false promise of transhumanism - a world of engineered permanent euphoria). And all pleasure is (apparently) subject to 'habituation' - pleasure fades unless there is novelty, or increased dose. This seems to be a biological built-in.
(This is why atheism so quickly devolves from pleasure-seeking - like the middle 1960s radical-leftist, libertine hippie ethic of promiscuous sex, hedonic drugs, and rock & roll... To the 1970s-and-onwards ethic of suffering-avoidance - with its hopes of a 'therapeutic' totalitarian world government that will prevent any kind of personal, social, planetary suffering. The change is from pleasure-seeking to pain-avoidant: from stimulant and euphoriant abuse to mass mediation with tranquillisers, mood-stabilisers, and antidepressants; from aiming-at a positive and pleasurable socialist utopia - to a society structured-around preventing the angst caused by global warming, microaggressions, racism, sexism, unbiological sexuality and sexual identity etc.)
Atheism, as it becomes mainstream, then mandatory (in public life) leads to rational and increasing despair; from which there can be no escape while atheism prevails.
The solution should be, but isn't, obvious: to re-examine our arbitrary and evidentially-unfounded assumptions regarding the fundamental nature of reality.
(These ultimate assumptions concerning the nature of reality are termed metaphysical.)
But once a person, or a society, is already-in the depths of atheism-induced despair; then it becomes increasingly difficult to make the metaphysical effort to escape, because real escape seems impossible, hence effort is futile and (here-and-now) counter-productive.
Despair is the fruit of atheism, and despair is perhaps the worst of sins; because it destroys the belief that escape is possible.
Hence mass human despair is the sin most deeply desired by the devil.
Note: In psychiatry - the highest rate of suicide - and of death from refusal of drink and food and lack of self care - is among the severely depressed: i.e. those who are in the grip of self-fulfilling despair. Captive animals in despair cease to reproduce despite protection from predators and adequate provision of food and shelter - precisely analogous to the modern atheist populations of The West.
Further note: If you join the same dots in a different way; you can see why atheism tends strongly to supporting the demonic agenda of subversion all good values; and opens the atheist to the ultimate demonic goal of value inversion (evil is virtue, ugliness is beauty, lies and fakes are truth) - in the sense that when value inversion becomes socially-expedient (as it is now) there is no strong reason to resist it, and many reasons to embrace it.