Why do people hide emotional pain?

Many people believe that showing too much emotions, especially if they are negative emotions, can make them appear weak. Those people hide their emotions in order to appear strong.

Many men were taught that real men must not display their emotions and so they hide their emotions in order to appear more manly. 

Introverts might not share all of their emotions and feelings with others. An introvert might choose to keep their emotions private and so not tell others about their emotional pain. (See The body language of introverts)

People who can't trust others easily and those who have trust issues might have problems expressing themselves. Those people might not share their emotions with others because they can't trust them.

Some people don't share their emotions with others because they don't want to annoy them. Those people believe that others can get annoyed when a person talks about their feelings.

Many shy people find it embarrassing to talk about their emotions and feelings. In such a case, those people choose to not display most of their bad emotions.

When a person feels inferior, they will want to feel superior by hiding everything that can make them seem weak. People who feel inferior might choose to hide their emotions in order to appear superior. (See What Causes an Inferiority Complex?)

Some people don't like to show their weaknesses to others or to tell them about their secrets. Those people might be afraid that those friends can become enemies one day then use the information against them.

Many people think that others might reject them or dislike them if they talked about negative things or if they expressed their emotions freely. Those people might choose to keep their emotions private.

Many people consider emotional sensitivity a kind of weakness. Those people might choose to hide their emotions in order not to let others think that they are sensitive.

Some people have created social identities that are strong, capable and in control. Those people might think that showing their emotions can ruin such an identity.