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As the title says, I'm looking for a word or idiom to describe a person whose mood changes regularly in a very short time and you never know what's going on in his head.

Some examples:

He can be very optimistic about the world and suddenly being extremely pessimistic after seeing some bad news in social media.

He can be very positive about dating a nice girl, yet being nervous because he "smells" something's wrong in the way she texts him.

He reckons for very long time about quitting his job and seem to be very determined. Once again, a little "maybe" occurs in his head and he change his mind suddenly.

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    Wouldn't moody, changeable, unpredictable, uncertain work? noun or adjective? It seems a broad question with so many answers. – haha Apr 8 '18 at 19:14
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    For me, even alive (lively) or human might work just fine as a synonym for someone with changeable moods and/or emotions. It is broad, because it's a judgment call. – Bread Apr 8 '18 at 19:29
  • My suggestions: I believe there are plenty of synonyms for an uncertain person. (personally, the one I like is inconstant). And for your idiom-request I suggest: A change of heart. "A change in one's opinion or feelings on a matter". E.g. 'She was going to take that job offer in California, but after thinking about how much she would miss her friends and family in Delaware, she had a change of heart.' Note that it is used usually (not always) from a negative to a positive position. – haha Apr 8 '18 at 20:17
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    Mercurial would work here. – user888379 Apr 8 '18 at 20:43
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a loose cannon TFD

one who has the propensity to act unpredictably or to lose their temper very quickly

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What comes to mind would be the term bipolar.

The individual described in the question would be referred to as suffering bipolar disorder, or would simply be referred to as bipolar.

Wikipedia defines bipolar disorder as:

Bipolar disorder, previously known as manic depression, is a mental disorder that causes periods of depression and periods of abnormally elevated mood.[...]During mania, an individual behaves or feels abnormally energetic, happy, or irritable. [...] During periods of depression, there may be crying, a negative outlook on life, and poor eye contact with others.....

Alternatively, the individual can be classified as having mood swings (which is a symptom of bipolar disorder). For more information, see here

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Let's be honest, unpredictable, while a simple word, is one that fits your two examples just perfectly:

likely to change suddenly and without reason and therefore not able to be predicted

Another possible suggestion could be the adjective volatile:

If someone is volatile, their mood often changes quickly (the idea is that they can quickly become angry).

Example: He had a volatile temper and couldn't have been easy to live with.

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This answer is not a diagnosis, but the symptoms as you describe them might correspond with borderline personality disorder. According to Merriam Webster Dictionary:

a disordered behavior pattern that has an onset during adolescence or early adulthood and that is marked by unstable, intense emotions and mood with symptoms including instability in interpersonal relationships and self-image, fears of abandonment, and impulsive or unpredictable behavior

Attribution: "Borderline Personality Disorder." Merriam-Webster. Accessed April 08, 2018. https://www.merriam-webster.com/dictionary/borderline personality disorder.

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