There are many causes of pain in one side of the eye and it is important to look at whether the pain is accompanied by other symptoms and the nature of the pain to help determine this.
If it is eyestrain or dry eye, the eye pain may also be accompanied by symptoms such as dryness and photophobia.
if it is an inflammatory disease such as conjunctivitis or keratitis, it is usually accompanied by symptoms such as increased discharge, dryness and tearing.
systemic diseases may also cause pain in the eye, usually accompanied by other systemic symptoms
For example, herpes zoster eye disease can also cause pain in one eye, but is more often accompanied by symptoms such as headache on the same side, orbital pain, pain in the temples and herpes of the skin on the face.