The Most Difficult Thing to Manage is Human Nature
Many people are always calculating their little contributions, yet they rarely see the immense sacrifices made by others.

They resent the small hardships they've endured, yet fail to see the deep wounds inflicted upon others. This resentment stems from their own internal scale, biased towards their own experiences, unwilling to acknowledge others' advantages. Consequently, their perspectives become distorted, and their own psychology also becomes warped and deviant.
It's simply jealousy when they can't stand seeing others having a little more oil in their bowls.

A small heart leads to big problems, while a large heart brings small issues. A correct mindset brings everything good, and a distorted one makes everything seem wrong.
Many things are actually caused by one's own unnecessary trouble – it's an unhealthy mindset.