I agree with you, but for a different reason.

I think a lot of people do not know the reason they are unhappy and attribute their unhappiness to any socially acceptable reason.

For example, during the American war on terror, a lot of soldiers came home with Post Traumatic Stress Disorder (PTSD). Once known as "shell shock", PTSD had long been considered a "honorably, even heroically earned" wound of war and its victims were shown great consideration and patience.

When shell-shock was renamed PTSD, the heroic connotation made it a "socially-acceptable" mental illness. Within a few years, people with all kinds of different mental health issues were diagnosing themselves with PTSD. Instead of lying, they were seeking the consideration, patience and lack of stigma that a PTSD diagnosis offered.

Stigmas began to fade as public education led to a better understanding of various forms of mental illness, and people began diagnosing themselves with anxiety, depression, ADHD, etc. It is very rare to find someone self-diagnosing themselves with socially unacceptable, highly stigmatized diagnoses such as sociopath, narcissism, psychopath, or severe, often violent, forms of personality and conduct disorders.

At the moment, mental illness is a trend, and Americans are busy trying to sound smart, by practicing psychiatry without a degree and diagnosing themselves, their kids, parents, celebrities and everyone else with the mental illness or personality disorder of the day.

That is the long way of saying, that I think a lot of people who cannot figure out why they are so unhappy, decide they must be mentally ill. After all, in a society where everyone else is so happy and things are going so well for them, only someone who is mentally ill could be unhappy.