Knowledge: therapy sessions with a psychologist and medicine, if needed.

Conversation:
How can you explain people can change emotional behaviors all of a sudden?
I am not sure about the answer to that, but speaking of mood, I can tell you about Bipolar disorder, which causes people to have periods of depression as well as periods with the mood elevated. Have you ever heard of it ?
I have little knowledge about it. What else do you know?
Its a disorder previously known as manic depression. It can cause periods of depression.
Oh, that sounds like a long period of time in pain. Do you know how to cure it?

Response: Yeah, therapy sessions with a psychologist and medicine, if needed, will recover from manic depression.