Crowding is the most common reason for teeth being crooked.
Crowding can be a result of the jaws not growing large enough for the teeth to fit in. Crowding can also be the result of the early loss of baby teeth that allowed other teeth to shift in a detrimental way.
Crowding can occasionally be due to unusually large teeth.
Crooked teeth as a result of crowding is the most common reason that people seek orthodontic treatment.