Mood Swings...

Why do moments of extreme depression follow moments of intense happiness?? Why do we get moody? Most important, why cant we explain why we feel the way we do? Lately I have noticed that I get more and more moody. It mostly happens when Im alone but still being sad and not understanding why just drives me crazy! The fact that Im getting more and more moody and that I have started to bite my nails again (a habit that I stopped when I was less than 10 years old) is setting off alarms in my mind. What's making me so nervous and stressed out that I am starting to bite my nails again? I used to do that when I was younger after the move to cairo in the gulf war...that was a bad time in my life so Im worried of what's going on unconsciously to make me get back to that habit. Is it work? Is it my life? Is it the people around me? I cant seem to put my finger on it...could it be that im too stable now so getting restless?