Despite taking regular care of my teeth, they have got stained and discoloured. Why is this?

Do you drink coffee?  Red wine?   Food and drinks that have strong color can stain your teeth. When you go to have your teeth cleaned they will polish them for you.

Certain life events can stain and discolor a person's teeth:
Some fruits and vegetables can also discolor teeth.
Coffee, tea, cola, and red wine are all well-known have the ability to produce a staining effect on teeth.
One of the most common reasons of teeth staining is smoking. Cigarettes, cigars, pipes, and even smokeless tobacco can cause tooth staining.
There are many diseases which can affect enamel.
The use of some medicines can cause tooth discoloration.
Antibiotics which can be deposited on enamel are tetracycline and doxycycline.
Some people drink water rich in fluorides. Excessive fluorides cam also cause teeth discoloration.
It is a normal event that a person's teeth will darken with age.Genetics. Some people have naturally brighter or thicker enamel than others. 
Locations where tooth decay exists are typically brown to black in color.
Some material used by dentists can also discolor teeth. Some of them are amalgam and sulfide containing material.
Teeth that have experienced trauma (like being hit in an accident) can discolor.

What shoud you do?  First, brush your teeth often…and the sooner the better. The less time you leave between eating and brushing, the less time the stain-makers have to do their dirty work. Second, eat lots of green leafy vegetables with stain-causing foods. As noted above, lettuce and similar vegetables can temporarily protect enamel by depositing a thin, protective film.
Simple techniques which achieves a most beautiful smile,or expensive options available today with cosmetic dentistry,from Tooth Bleaching,Veneers,Crowns,to tooth reshapings,macroabrasion and microabrasion procedures
There's Much More Than A Smile: What You Should Know About Caring For Your Teeth

It is difficult to cite a single reason for this. But in general, you could say that the intake of hot and cold beverages including alcohol and soft drinks combined with smoking do affect the whiteness of our teeth. Also, as a person ages, teeth tend to discolour as well. To get rid of this problem you can opt for the varieties of tooth whitening products that are available in the market.