Curly hair is caused by asymmetrical hair follicles. It is also caused by an uneven distribution of keratin along the hair follicle. In the case of curly hair, the keratin gathers near the inside edge of the hair, and the hair curls around it.
Individuals with straight hair have symmetrical follicles, so the hairs they produce are round and straight. Curly hair follicles are flat, while straight follicles are actually round.
Whether someone's hair is curly or straight is determined in part by genes. However, just because someone is born with one type of hair doesn't mean it will remain that way. Hair can become curly or straight in time because of hormones or, in rare cases, medications. Hormones change during puberty, pregnancy and menopause; these changes can actually alter the shape of the hair follicle.
Graying hair also changes texture. As hair grays, hydrogen peroxide builds up in the hair follicle. This can make hair more thick and curly.
Over-processing hair can actually prevent straight hair or curls, even if hair is naturally wavy or straight, because it prevents hair from responding to any kind of styling treatments.
Unfortunately, chemotherapy treatment can also change the hair follicle and make hair curly. This is because the drugs administered in treatment inhibit receptors that regulate hair growth on the edge of the follicle.