The two main reasons teeth crack are linked to diet and environmental factors.
Sugars in our diet provide fuel for the bacteria that live in our mouths. If these bacteria are not removed on a daily basis the acid by-products of the bacteria will begin to destroy tooth enamel. This is the beginning of tooth decay. When decay progresses, the enamel is weakened causing a cavity. A small cavity is similar to a small chip in your car windshield: the longer you leave it, the greater the chances the chip will spread and become a crack. A diet high in sugars combined with poor oral hygiene habits, cause more acid damage and more weakening of the tooth. Eventually, teeth can crack.
Teeth also crack because of the force that is placed during chewing and grinding. The human jaw has been shown to produce on average over 150 pounds of chewing force. If the tooth has already been weakened by decay or has a small crack in an existing filling, the damage can deepen due to the daily force that is placed on it.
Injury can also cause a tooth to chip or crack. A sudden, excessive force placed directly on the tooth may cause damage. This can occur with contact sports and accidental injury situations.