In childhood, some people were conditioned that it wasn't safe to express their emotions.
For instance, at the first sign of any tears, they would have been beaten, shouted at or got told to "...shut up or I'll give you something to cry about."
Now as an adult, they have, locked off their feelings, don't know how to express their emotions healthily or have decided not to share their opinions in fear of a backlash and confrontation.
In the worst case scenarios, they show lack of empathy and compassion in their behaviour towards others causing massive amounts of pain.