Tooth sensitivity can be caused by a number of things. An unnoticed cavity or abscessed tooth can sometimes be a culprit. But over time and as you age, changes in temperature, as well as such behaviors as tooth grinding (bruxism) and overly aggressive brushing, can cause small, often microscopic cracks or fissures on the chewing surfaces of your teeth (or near the gum lines), which exposes the inner structures called dentin.
Hypersensitive teeth can cause people to change their eating habits, avoid social situations, or even avoid proper oral hygiene because the simple act of brushing or rinsing causes pain. Relief can sometimes be had by using a desensitizing toothpaste, sealants, or special fillings.