You're being too rough when brushing.
We're all in a rush, but don't cut corners when brushing your hair. It's essential that you're gentle throughout this process, especially when you're working with wet strands. "Water weakens the bonds within strands of hair, removing up to one third of its strength," explains Skjoth. "Wet or damp hair can therefore be more easily broken, ripped, or torn out when combing or brushing." We like using the Wet Brush ($9) for postshower styling, as it has flexible bristles that won't tear out hair.
Furthermore, rough tugging on knots will weaken and possibly permanently damage your hair follicles, which are responsible for growth. Work on small sections, starting at the tips of your strands and working your way up as you detangle. If you have long hair, it's even better to hold your hair tightly at the midlengths while brushing so any tugging is absorbed by your fist, not your scalp.