According to Fluoridealert.org, white spots or streaks on the surface of teeth are caused by fluorosis. Fluorosis is the result of an overexposure to fluoride from an early age that causes dental enamel to become more porous. In its more severe forms, fluorosis can cause teeth to erode.
Fluoridealert.org states that even mild cases of fluorosis can lead to self-esteem issues for adolescents. The risk of fluorosis is amplified by consumption of fluoridated drinking water and ingestion of fluoride toothpaste. Foods processed with fluoride can also increase the risk, says Wikipedia. The discoloration left by fluorosis cannot be remedied by brushing one's teeth, according to Fluoridealert.org, although Wikipedia notes that specialized cosmetic procedures can be effective in treating it.