Why does the inner corner of my left eye hurt so?

Need exam to tell. No way to know without a thorough eye exam. Possible causes include infections, inflammation in the eye or eye muscles, eyestrain, sinus inflammation, optic neuritis, among others. Start with a thorough eye exam by an ophthalmologist, then consider sinus x-rays, ent, or neurology consultation if the eye exam does not reveal a cause for the pain.