Why do humans live with closed hearts?✶
The answer I'm about to give you reminds me of the Tin Man (without a heart), the Straw Man (who had a problem with the substance he was made of) and the Cowardly Lion (who was scared of everything) — in the movie 'The Wizard of Oz'.
They represent the three flaws in humanity that cause them to live with closed hearts. (The closed heart is represented by Tin Man who was hollow and didn't have a heart.)
Humanity's problem begins with the inferior substance they're made of (Straw Man). Humans are made of atomic matter that causes them to be transient beings that die after a certain number of years. Their survival depends on how well they perform their duties — often at less than adequate pay. Their short lives are mercilessly dominated by mortality and endless duty. The fear — Fearful Lion — of death or failure to work hard or well enough, drives them mercilessly through their short lives.
The fear of failure and death that can strike at any moment, closes their hearts. The belief that they must be tough to get ahead in this world, further makes them suppress tender or loving feelings.
✶See: Causes of a Closed Heart