Tooth sensitivity is usually fairly straightforward: your teeth hurt when you eat or drink something cold or hot, or when you brush or floss. In some cases, the cause is a cavity. In others, it's a cracked tooth, a gum problem, or a tooth that's lost much of its protective enamel.
The cause could be as simple as brushing your teeth too vigorously. If you have tooth sensitivity, see your dentist. He or she will examine your teeth and gums and can prescribe a desensitizing toothpaste, which contain compounds that block sensations that make your teeth hurt.