"Moodiness" or "the blues" can also be caused by adverse life circumstances / personal life challenges / academic challenges / job- or career-related stress / financial adversity etc. Are you experiencing any of this at this time ?
Or, as Yigeren pointed out, you could be depressed. If there is nothing adverse in your young life at this point, then it is may be advisable to contact your doctor to be evaluated for depression.
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O villain, villain, smiling, damnèd villain!
My tables—meet it is I set it down
That one may smile, and smile, and be a villain.
At least I'm sure it may be so in "Denmark".
-- Hamlet, 1.5.113-116