Um....illness?
If you've got tremors, it could be a neurological problem, don't you think? Maybe not enough seratonin? Maybe you just need to eat a sandwich - do you have diabetes or low blood sugar? Hypoglycemia?
On the other hand, you might just not be getting enough sleep or you might be like the Cowardly Lion who was scared all the time.
Really, if you are shaking badly enough that people notice, it's time to get a doctor's opinion.