There are many many, many, MANY reasons and one post cannot cover them all. Not to mention that there are still worlds yet to be discovered in the deep and complex world of human emotions.
But a few thoughts...
First of all - the human psyche is very fragile. One thing goes awry and the whole system is thrown out of whack. Many people are already born with "stuff" and then life's ups and downs, traumas etc. etc. further complicate matters.
Another reason: We are descended from people who lived through unthinkable suffering. Pogroms, oppression, hunger, unbearable cold, disease, countless sources of fear, forced long term conscription to foreign armies, high infant mortality etc. etc and most significantly - the Holocaust.
How does one remain sane under such conditions? The answer is to suppress emotions. That is how to deal with the impossible.
So we are descended from people who [by no fault of their own] had to suppress emotion. This has tremendous negative ramifications on the emotional level. The children of people whose inner world is tortured suffer tremendously. And then they pass this on to THEIR children.
This impacts our emotional world until and including this very day.
But there is HOPE! We can learn to become emotionally healthier and healthier. If we neglect our emotional health, however, not only will we pay a high price but so with all those around us.
