Curly hair occurs when the hair follicles angles into the dermis which causes the hair to curve as it grows which then leads to a person growing curly hair.
A persons curly hair has more of these bonds, referred to as disulfide bonds, than straight hair does.
Some people are born with curly hair and then later after the toddler years the hair can become straight again.
My brother was born with really curly hair and he got more curly when he was a toddler and it was so cute on him and now he has straight hair as an adult.
Not all curly hair will change to straight hair as a person grows but it can if it's mean to be.
Curly hair if born with it can be genetic and if your parent or parents had or have curly hair then it can result in the child being born with curly hair as well.
People also have different shapes of hair follicles which contribute to the shape of their hair and if you have straighter hair follicles then you will have straight hair but if your hair follicles are not straight then curly hair is usually the result.