The best way to really get an idea of why you want to do optometry is to probably talk with an optometrist, or shadow/observe one and especially during an exam.
I had a hard time thinking about why I wanted to do optometry until I shadowed at my optometrist. What I saw was the personal interaction between the optometrist and the patient, and the deductive reasoning behind prescribing corrective lenses for a patient. So aside from being a stable profession, and flexible hours, the what I saw fit what I wanted to do which was help people in my community as well as be challenged by the complexity of the eye and various ocular health conditions.
What stemmed my interest was the technology used to check our eyes, and something I have the pleasure of doing is take retinal photos of patients in our office, and actually SEEING results of diabetes, high blood pressure, and cataracts.
So, my reason for optometry isn't related to some eye disease I have or whatever. But I see it as a profession that will provide exciting challenges, while at the same time giving me an outlet to help others. So, as corny as it sounds, I don't think it matters what answers you give to the interviewers, as long as you do some soul searching and understand why you want to do optometry and present it to them honestly. Cause if there is any weakness in your answer. They will dig into it, at least that's what happened at SCCO, but I was still accepted.