What causes someone's eye to twitch?

Eye twitching is as a result of spasms (contractions without control) of muscles around the eyelid. There are many things that can cause the eyes to twitch. These include: fatigue, injury to the cornea, blepharospasm(movement disorder of the muscles around the eye), dry eyes (decrease tears production), light sensivity, entropion (inwardly turned eyelid), smoking, excessive alcohol and caffeine intake, stress and glaucoma.

It can be a sign of a brain disorder like: Bell's palsy, multiple sclerosis and tourette syndrome.
Most of the time, eye twitching is mild and they do go away on their own.