Chemical Imbalance - whether it's hormones (PMS, PCOS or PMDD) or something like depression/bipolar.
Puberty - newly developed hormones raging in teens
Sleep apnea - a sleep disorder where a person stops breathing repeatedly during the night
Those are the three major causes, but there are a few more:
ADHD, Pregnancy, heroin addiction, ALS, Huntington's Disease
and several others which are listed at this link:
http://www.wrongdiagnosis.com/symptoms/mood_swings/common.htm