Bad moments are states that start and end and have a reason for such a feeling. They last a few days and they don't interfere with the daily activities. One gets over them naturally without any special action.
On the contrary is not necessarily related to a particular event or to a present situation.
Sometimes we don't even know why we feel that way, depression is not a unique effect illness, nor does everyone suffer from it in the same way, according to each one's personality, the manifestations may vary, the process and time needed to recover from the illness