Our tooth enamel, like our fingerprints, is unique. No two people have exactly the same color and porosity. There are many reasons our enamel becomes stained and more yellow. Some you can control; others not so much. Most stains are on the enamel, the outer protective covering on the teeth. Some, especially the result of medications, can be on the dentin (the bulk of the tooth beneath the enamel) and are harder to address.
These factors can impact the whiteness of your teeth:
What you eat — Foods such as blackberries, blueberries, and raspberries have natural pigments in them that stain the teeth.
What you drink — Coffee, tea, red wine, some juices, and colas also stain your teeth.
Smoking — Nicotine leaves yellow stains on the teeth.
Drugs — Tetracycline is the main culprit in stained dentin. This antibiotic used heavily in the 60s and 70s left the teeth of many children who had taken it stained. Sometimes this staining is so deep and pervasive that the only way to address it is with bonding or porcelain veneers.
Fluorosis — If a child has too much fluoride, it leads to fluorosis, which stains the teeth. This is very rare.
Your genes — Each of us has a different shade of the enamel on our teeth. Some is more yellow than others.