To understand why a cracked tooth hurts, it helps to know something about the anatomy of the tooth. Inside the tooth, under the white enamel and a hard layer called the dentin, is the inner soft tissue called the pulp. The pulp contains the tooth's nerves and blood vessels.
When the outer hard tissues of the tooth are cracked, chewing can cause movement of the pieces, and the pulp can become irritated. Eventually, the pulp will become damaged to the point that it can no longer heal itself.
The tooth will not only hurt when chewing but may also become sensitive to temperature extremes. In time, a cracked tooth may begin to hurt all by itself. Extensive cracks can lead to infection of the pulp tissue, which can spread to the bone and gum surrounding the tooth.