Sour grapes (the fox who convinces himself that the grapes highe up he cannot reach are sour, the employee who beleives that working hard is a great idea erc.) is irrational, but can be very useful psychologically.
Beleiving in work ethics makes the lives of many people much happier.
I believe very much is self delusion. Psychologists found that positive illusions ar good for mental health. And reality is not so important, yet psychological feelings about ones situation etc. Is crucial to his happiness and mental health.
The one who successfully convinces himself that he is well off will be happier and healthier.
only loss is that it leads to practical mistakes (he will not try to improve s objective situation). This loss can be much lower than the psychological and health value of believing he is in great shape. (beliefs have a strong effect on happiness and health. Satisfaction with some parts of life etc. Is more important to happiness than actual state)
If the psychological income is higher than the loss, it is very rational. Similar to self illusions that contribute to happiness. Talking about "being rational" here is nonsense and irrational.