Because people are threatened by their own suppressed pain, which they really don’t want to experience. When you cry in front of other people, they are afraid they will start to cry with you. They don’t want that - they want to maintain their pretend happiness and positivity for as long as possible.

A person who has no pain of his own will be perfectly comfortable around you crying. Unfortunately, such a person is very rare.

Don’t let others prevent you from crying. Cry, cry all you want! If they don’t want to see that, it’s their problem, not yours. Tell them that if they don’t like it, they can leave you alone, but right now you just want to cry.