Why would one eye always be dry but not the other?

Mostly anatomical: Dry eye is very common, but differences between the eyes less so. One eye may have an incomplete blink pattern or the opening between the lids may be wider leading to exposure. Eyelid problems may worsen one side. There may be partial blockage of tear drainage on one side which would help to moisturize that side. There may be decreased tearing on one side in a condition called horner’s syndrome.