How can you lose your eyesight?

By doing things like reading in the dark and (if you already have glasses) not wearing them or wearing them when not needed. Also, it can just be genetic.

Also staring in bright light for long and not wearing sunglasses in the sun, looking at brightly/sharply reflecting objects, using your phone or computer in insufficient lighting and not eating right foods that improve or make your eyes better: carrots, broccoli and Brussels sprouts, sweet potatoes, spinach, turkey, sardines and more. These foods provide vitamin C, Omega3 fats and zinc (plus the B-vitamin niacin, which specifically protects against cataracts).