Stress, strain. Eyelid twitching is an annoying occurence that is thankfully temporary and harmless. Often related to stress and fatigue, a small portion of the eyelid muscles become "over-excited" and create the twitching that you feel. Although rest, cool compresses, and artificial tears have all bee attempted, the condition runs its course on its own. If you are having forceful twitches, see an eye doctor.
Often eye irritation. Eyelid twitching can be the result of many different things or sometimes the cause cannot be idenitified. Common triggers include decreased sleep, overall fatigue, stress, increased caffeine intake, smoking, eye irritation or inflammation, dry eye, eyelid inflammation or bright lights. This is so very rarely due to a medical problem of consequence and almost always goes away spontaneously.