What causes hair loss?

My girls were roaming around without getting their hair groomed for a long time. When I noticed it almost by the end of the day, I immediately called them to come to me with a brush so that I can groom it properly. Getting overly engrossed with what they were doing, they simply refused to come to me.

Me: Girls! If you don’t come now, then it is going to be a big loss for you and not for me.
Girls: How?
Me: Just stop asking questions and come to me now.
Shruthi: Are we going out now?
Me: No. We are not.
Swathi: Then we can as well do it tomorrow.
Me: Why don’t you want to look presentable while at home?
Shruthi ran to fetch a comb and came to me. Being excited over her action, I began brushing her hair. The knots that got accumulated in her hair gave her enough pain but she still did not stop talking.

Shruthi: What will actually happen if I do not brush my hair?
Me: As you now see, lots of knots will get accumulated. Not only that, your hair will get badly damaged and you will lose hair little by little.
Shruthi: Knots, I agree but do not talk about hair loss.
Me (curiously): Why?
Shruthi: Whenever I have left my hair loose, I have never seen hair falling on its own to the floor but the moment you take a comb and start brushing, I see that hair gets curled up onto all sides of the brush. So hair loss is only due to combing.
Me : !!!!????