Could you explain why does it hurt to brush my teeth twice a day?

Root exposure?: You might have some exposed root surface down along the gum line or you might have a small cavity. See a dentist to get this checked out.

Tooth brushing pain: Depends where. Along the gum can be exposed tooth root, or a gum infection. Depend on the force of your brushing. Brushing hard does notclean better. Smaller amounts of toothpaste is better than more. Mechanical toothbrushes are meant to glide slowly over the teeth. Don't actually brush. Best time to clean your teeth is just before sleep.