Why don't people understand themselves when they' re sad

Why when sad, no one knows himself?
Even if your heart is in chaos, but in the eyes of others, you are just a little more silent than usual, no one will feel strange.
This kind of war is destined to be single-handed.
So why not even the people who care about themselves?
You laugh, the world laughs with you, you cry, you cry alone. People only expect your happiness, but don't bother to understand your pain.
This is the world of snow, each with its own obscurity and brightness.