Could you tell me why my left eyelid keeps twitching?

Benign. Twitching of the lid is common in the middle years. It is random and not precipitated by any known pathology. Mostly it is subtle and you can feel it but others cannot see it. Rubbing the twitching lid commonly stops it. Some think extra calcium and magnesium cut down the incidence. Be assured that it is benign and will not harm the eye or vision.