It is a human ability to self delude in order to feel better about oneself. It has been found that the depressed are generallly more realistic about their lives than the happy ones. No doubt we should be idiots to want "realism" and get depressed. (unless one is religiously "truth seeking" which is yet another mental illness).
So for some issues most people have a very strong interest to beleoive in what they think is reality.
Two great examples are marriage and kids.
Whether these are good or not is a complex question. (partly because nobody feels comfortable to think lucidly about these, Or it is human nature to get confused here). [The available sceintific data holds that marriage is variable good for some and bad for other in the long term, while most enjoy it in the beginning. Kids are not adding to our happiness usually as far as we can measure happiness and life satisfaction]
Yet those who are married have a strong self interest in beleiving that marriage is crucial for life and happiness. I witnessed it yesterday when visiting an acquitant who is laboring madly with her kids and job. I suspect she was more alive nad happy when single. SOmehow I heard her opinion that the single persons are highly unhappy. I was unable to avoid the thought that she has a vested interst in this beleif.
Same for kids. The majority of people who do have kids (many out of carelessness in sex), love their kids (usually) and cannot change history. Their best psychological tactic is to beleive in a hardcore way that kids are a great thing. (When some one says that the only thing in his life are his kids, cannot avoid the suspicion that he simply does not have anything else in his life. I have heard these words only form people wghose life seems to have ended long ago. But who am I to make fun of the last consolation of the depressed)
Actually, the only problem with these delusions are when it comes to decisions. And when the masses delude themselves it also creates a public opinion that is highly deluded.