Will make a person's life completely collapse.
I wonder if it is the classmate of the questioner. Parents or people around them suddenly become selfish, or they may not.
This is normal, because everyone wants to be a perfect person, some people don't necessarily. However, people will keep the bad side in their hearts and vent it when people in front of them are very familiar with it. Selfishness is disgusting, and it will make interpersonal relationships worse.
I have selfishness, too. Everyone has selfishness.
However, there are also smart people who will release their selfishness at the right time and place and can grasp their selfishness.