Do you cringe every time you eat something cold, like ice cream? There are many things that can factor into tooth sensitivity. One of the major causes is tooth/gum recession, this is when your gum tissue recedes and leaves the root surface of the tooth exposed. The root surface of your tooth does not have the hard outer layer, enamel, of tooth to protect it.
Reasons why you may have gum recession can start with something as simple as brushing to hard. You should always brush your teeth gently with a soft bristled brush. Being too aggressive with brushing can wear away gum tissue.
Another Factor could be gum disease/periodontal disease. This is when you have an active infection in your gum tissue. The bacteria and infection in the tissue can cause bone loss which in turn causes gum recession.