It has been noted in various studies that people with higher IQ tend to be more depressed or unhappy with life.
The intelligent people tend to look at the bigger picture in life, and there they start to ask the basic questions revolving around humanity and individual existence.
They try to find meaning in this absurd and chaotic world on the whole, which they cannot control much and this is the reason for their sadness.
Another factor is socializing. Intelligent people tend to spend quite some time with their thoughts in solitude, hence they socialize less than average people.
Therefore, generally speaking, intelligent people have less social interaction compared to average people. This hinders their emotional development as they rarely find someone who can think on their level and have mutual emotions towards things. Smart people are also rarely interested in other people’s life.
This also leads to low key social life, which can also feel lonely at times and lead to sadness or depression.
Not all intelligent people are appreciated for their smartness. Many super-intelligent people, like Albert Einstein, do tend to achieve fame and success.
But mostly, intelligent people see average people being more appreciated than them in the social aspect of life. This is another reason intelligent people might get depressed.
Ignorance is bliss, but smart people are rarely ignorant. Socially the smart people are also isolated from a group, for example, in a high school nerds will be on the lower side of the social order, isolated from the famous kids. Because such people also think a lot more, they tend to get emotionally disturbed over small things.
They also prefer to run things their way if they know what they are doing, and if they are resisted, they feel unappreciated. Sometimes average people are intimidated by smart people because they feel somewhat inferior to them intellectually.
The things smart people talk about are also unknown to the average person to some extent and hence deemed as boring. Smart people might also develop superiority complex, making their ego too big to enjoy a happy social life.