I was born with cataracts. My eye doctor removed one when I was a child, and waited until I was in my 30's to remove the other. I believe the reason they only remove one, is to have another eye to operate on in case something goes wrong in the future with the other operated-on eye. This is the case with my eyes. Eyes become weaker after being operated on. With cataract removal surgery, there can be retinal detachment, or high pressure in the operated eye years in the future. Maybe that is why your vet is only going to operate on one eye. Also, the eyes will have to be patched. Please ask your vet.