Its a disorder previously known as manic depression. It can cause periods of depression.
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?
Bipolar disorder, previously known as manic depression, is a mental disorder that causes periods of depression and periods of elevated mood.