Sudden sharp stabbing pain in eye for no reason. Left eye. What is this?

? Dry eye. Your description sounds like dry eye. Try using frequent artificial tears during the day, ointment at night, warm compresses, and fish oil supplements. If no improvement, see an eye doctor.
Common. Sharp needle-like pain lasting a few seconds is a common complaint with no known cause. If associated with headache, or persisting for more than a few minutes, then it may be sinus-related or a variant of migraine raeder's, eg--cluster headache variant).