Pain in or around the eye can occur for many different reasons. The pain ophthalmologists are most concerned about is pain at the TEMPLE, sometimes associated with pain when you chew food or move your jaw, and sometimes associated with tenderness at your scalp. If you have these symptoms, you should see an ophthalmologist immediately, especially if you are over age 60. These are signs of something called "temporal arteritis". If you do not have these symptoms and simply have a dull ache over your left eye, this could be sinus related. Sinus cavities exist below, behind, and above the eye and sinus congestion or inflammation can manifest as pain around the eye. Do you have a history of sinus problems or symptoms of sinus congestion or allergies?
Even cervical neck pain - arthritis, muscular pain, etc - can result in something called "occipital neuralgia" which causes pain around the eye.
Sometimes a run-of-the-mill headache can cause this.
If your pain is mild, not debilitating, and not associated with the symptoms I described earlier, I would not worry.
If it persists, see an ophthalmologist.
Please note that this forum is for general advice only - it is not a substitute for an eye exam by a licensed professional. It is not possible to accurately diagnose or treat any medical condition without a complete history and exam.