Tooth sensitivity can have several different causes. If you are noticing sensitivity in your teeth that you haven't had before, it is a good idea to have your dentist check for possible causes.
Sometimes sensitivity is caused by something simple such as gum recession or inflammation. Sensitivity can also be a result of tooth trauma (ie. biting on something hard, an accident, or grinding and clenching) or a developing cavity. There are times when sensitivity is a sign of something more significant such as an infection, a tooth fracture, or other oral pathology.