Straight hair are due to the follicles that produce round strands whereas curly hair are due to follicles that produce oval strands. Curly hair is oval in cross-section while strait hair is round. Some people have curly hair and some have straight hair because we are different. Our eye color, hair color, skin tone, and if our hair is curly or straight are genetic.
One theory also tells that straight or curly hair is due to the temperature in which their ancestors have lived. Straight hair is a sign that the person has lived in a cooler climate, because it will lay flat to conserve body heat. Curly hair is known as a heat coil, meaning that it disperses heat more rapidly. Pepole from arctic regions have straight, oily hair. People who live in desert regions like the desert or in africa have curly hair to disperse the heat.