Why is the mood very low, but I just can't be happy. The heart was thrown into the ice hole?

What is the reason? Is it inexplicable sadness? Maybe I feel cold sometimes. Listen to some songs, play some games, run some long-distance races, and chat with people close to you ... In short, there will be ways to make yourself happy. Because happiness is a day, and unhappiness is also a day. Why not choose the former?