One of many reasons for low vision is nearsightedness, which means you can focus better on near objects than on distant objects. Over 100 million people in the US are nearsighted but less than 20% are born that way.

Causes
Genetics
Age
Stress
Excessive close-up work
Personality type
Diet

Self Help Prevention
Manage chronic stress
Eat healthy
Maintain a regular, aerobic exercise routine
If you are a computer user, review Computer Fatigue Syndrome

See Drugs That Harm the Eyes for potentially harmful drugs.