As you no doubt already know, looking into the sun can cause permanent damage to your eye.

I don’t need to look into the sun to have my eyes hurt from the light. If I have been in the dark, coming to the light hurts whether it is a bright fluther page or lightbulb causing it. I can’t open my eyes wide open, but I usually can close my right eye and my left does not cause as much pain. I have no idea why. Interesting question. I am pretty sure if I close the left and try to see with the right I am still in pain. For whatever reason my right eye is more sensitive to light. Is it the same for you? You favor a particular eye for bright light?

I didn’t answer your question, I hope you don’t mind my answer. I’ll be following.

It seems to be a summation effect. The pain is “virtual pain”, not caused by actual damage, but more a prompt to turn away from a potentially dangerous stimulus. The eye itself doesn’t hurt (though it may later if the exposure is severe enough), but the additive effect of the two eyes lowers the discomfort threshold and triggers the avoidance response.

I’ve noticed a similar phenomenon with hearing: If I adjust the volume on my headphones to an uncomfortable level for both ears, I can listen to the same level through one ear without discomfort.

Because you’ve gone blind in the other eye.

Do not look at the sun. Ever hear the directions when there is an eclipse? Much danger to your eyes, really, and you want your eyes to be healthy and not filled with spots.