To appear strong
Many people believe that showing too many emotions, especially negative emotions, can make them appear weak. Those people hide their emotions to appear strong.
To appear more manly
Many men were taught that real men must not display their emotions, and so they hide their emotions from appearing more manly.
They are introverts
Introverts might not share all of their emotions and feelings with others. Introvert might choose to keep their emotions private and so not tell others about their emotional pain. (See The body language of introverts)
They have trust issues
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.
Not to annoy people
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.
They are shy
Many shy people find it embarrassing to talk about their emotions and feelings. In such a case, those people choose not to display most of their bad emotions.
They feel inferior
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 from appearing superior. (See What Causes an Inferiority Complex?)
Not to be used against them
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.
Not to be rejected
Many people think that others might reject them or dislike them if they talked about negative things or expressed their emotions freely. Those people might choose to keep their emotions private.
Not to seem sensitive
Many people consider emotional sensitivity a kind of weakness. Those people might choose to hide their emotions, not to let others think that they are sensitive.
Not to lose their social identity
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.
Body language: Crossing the arms meaning? Why do people fold their arms while communicating with others?