My left eye keeps twitching. What can cause that?

Myokymia. Usual cause is orbital myokymia, a rather benign disorder associated with fatigue, eye strain, electrolyte imbalance, lack of sleep. See an eye specialist and get this addressed.
Many possible causes. Your twitches may be related to any number of reasons including stress, stimulant overuse (for example, caffeine), overexercise, or nutritional deficiencies. See your physician if you are concerned. Here is some more information: http://www.nlm.nih.gov/medlineplus/ency/article/003296.htm.