There's a range of reasons teeth become discoloured. Genetics, your environment, even taking certain medication can all have an impact on the whiteness of your smile.
The most common reason is surface stains on your enamel. This is down to everyday food, drinks and lifestyle choices. Over time, these stains can work their way to the layer below enamel, known as dentin. When this becomes stained, the whole tooth looks darker because of the translucency of enamel.