Your important documents have been misplaced and you got sad.
Your motorbike has been stolen, you are sad,
Someone misbehaved with you, your mood is off and you are sad,
She lost her father or mother and she didn’t like to go for work,
Above stated all events cause sadness, however after certain time, we get over it and move on.
This is normal, it mean you are a healthy person and you didn’t have a mood disorder. But if you are sad without encountering any such event and you become sad most of the day, you feel irritable and tired. Your eating habits and sleep manners has been disturbed, then you are suffering from a mood disorder. We got depressed thinking that we are the only one with that disorder.
If you are feeling being isolated, lonely and seclusion, disappointed and despair then these are the symptom of depression and hence depression is a mood state, the most prevailing psychological disorder and leading cause of the disability.
You might like to read , Difference between Manic and depressive episodes: Clients of Bipolar I Disorder experience both depressive and manic episodes. How to differentiate between manic and depressive episodes?