Why do females pull out their own hair?

i dont understand why women do this. when i was 15 i had a 13 year old girlfriend later in life became my baby momma but the whole 8yrs we were together she would pull out tiny strands of hair and eat the white skin at the end, now im 28 years old and i have a 35 year girlfriend that scratches her head all the freakin time and i watch her pull out her hair and i tell her to stop and she says it itches, she spends all kinds of money for these scalp treatments and shampoo treatments and i still aways see her digging at her head, i have actually looked at her scalp, she has about 15 or 20 tiny red sores on her scalp and her 8 year old daughter does the same thing but she only does it like when she is sitting and watching tv or playing on the computer, we have to tell her all the time to get her hands out of her hair, her daughter is doing better because about this time last year she was 7 and was completely bald on the top of her head, and she has really beautiful curly hair so she kinda looked like bozo the clown, i know it is not nice to say that but thats the only way i could explain how bad it is. i just dont understand what makes a person reach into their hair and pull it out.