It can be cause by a loss of identity, or not seeing and accepting oneself and flaws.
This relates to suicide committed by teenagers who only see themselves as one thing and are unable to view themselves as more than that. For example, intelligence is the only thing some people see themselves as and nothing more.
For adults, I would say it due to societal pressure of being perfect and having a perfect life or not being in the position you want to be in; seeing yourself as a failure and unaccomplished.

Freedom is accepting the world as it is and making the most out of it.

Intelligence does not mean you have wisdom. A requirement for philosophy. From my experience, some or most of the smart people I have encountered are individual with a narrow mindset moulded by our society. A trait which great philosophers do not have.

We want to believe that we can change our situation/life therefore our choices need to be matter. We need to believe our life has not be pre-determined. That we can choose the situation we are in. We want-need to be in charge of our life.

To me, our purpose in life is determined by us and is discovered as we go through life. For me it would be survival and being my best, it may not
Fancy, but it provides me a reason to keep going.