This is a very common problem and it is characterized by very round and noticeably discolored enamel surfaces. It occurs as a result of exposure to abrasive materials like fizzy drinks and sugary snacks. The acid in these ingredients wears down the enamel and gives it a stained and unhealthy look. In some cases, restless brushing may also be a factor.
The one thing that will lead to enamel degradation faster than anything else is sipping on fizzy and sugary drinks all day, every day. This includes sports drinks, carbonated soft drinks, and most forms of wine. In fact, this is considered to be a professional risk for professional wine tasters and reviewers.