The short answer: heredity. Just as you inherit other characteristics of your parents (she has her father’s eyes), you inherit genetic instructions that determine physical aspects such as the size and shape of your jaw. Sometimes people with narrow jaws have less room to accommodate their full set of teeth without some overlapping.
Of course, physical development is nearly always a result of both nature (heredity) and nurture (your environment after you are born). That means that crooked teeth can also be attributed to events such as losing your baby teeth too early, jaw injuries and diseases of the gums and jaw. There is some evidence that prolonged thumb sucking and pacifier and bottle use beyond the age of three may contribute to misaligned teeth.
Today, orthodontists offer a number of techniques effective at straightening teeth and giving nearly anyone the smile they desire.