The teeth is covered with a very hard enamel which provide protection to teeth from physical, chemical, bacterial and thermal insult. It is a dead covering which protects the underlying dentine.
But dentine has tubules and nerve ending and is much softer than enamel.
So, if the dentine is exposed by some defect in enamel continuity, you feel sensitivity.
The dentine can be exposed in various conditions like
-fracture of crown
-wrong and excessive brushing.
-night grinding of teeth
-erosion of teeth by certain acidic food
-recession of gums around neck of tooth due to plaque and calculus.
-certain genetic reasons when enamel formed is defective.
and many more.
Please visit your dentist so that he can identify the main reason and provide appropriate treatment. Do not try desensitising paste available over the counter of chemist as they will mask the sensitivity but will not reverse the reason for sentivity.