Tooth sensitivity is usually caused by dentin in the tooth exposed by receding gums or periodontal disease. This is a very common phenomenon, and has a high percentage.

When exposed, the tooth does not have a layer of enamel like the crowns. Instead, the root has a thin wrapper layer outside called move-shoots. When move-cement loss, dentin will be exposed.

Brushing too hard or using toothpaste with abrasive wear can also cause the enamel surface and expose dentin. A diet rich in acid - such as sour foods, pickles or soda - can cause tooth erosion and decay the teeth and lead to expose the dentin surface.

When tooth sensitivity, you will feel pain when brushing teeth. So, if you brush your teeth carefully because of the pain, you will face more risk of tooth decay and gum disease. 
Normal sensitivity due to tooth decay, but you do not know, old worn teeth, receding gums, root exposure, ...

Reasons that cause pain in the dentin was exposed? 
Dentin contains thousands of tiny ivory tube which can only be seen with a microscope. These ivory pipe connection from the tooth surface, through the dentin to the nerve center of the tooth pulp.

These ivory tube containing fluid. After eating or drinking hot or cold foods, fluid in these tiny ivory pipe to move and stimulate the nerves in the tooth caused toothache. 
The difference between tooth erosion caused by teeth grinding, abrasion and erosion: 
- Track by gritted teeth: tooth wear caused by contact between the teeth with 
- Abrasive: wear due to foreign material in contact with the teeth like bristles, nails, hair clips, ... 
- Erosion: exposure of a chemical agent to the teeth, such as lemon juice, gastric, ...

Prevention of tooth sensitivity: 
You can reduce the risk of tooth sensitivity by oral hygiene to prevent receding gums and periodontal disease. Brushing and flossing properly use under the guidance of a dentist. And use low-abrasive toothpaste can help reduce the risk of tooth sensitivity.

A diet acid also helps prevent tooth sensitivity. 
Failure to treat sensitive teeth can lead to other oral health problems, particularly pain when brushing will prevent you comb carefully - this is the risk of tooth decay or gum disease.