Every time you move or touch your hair, the cuticle layers of each strand rub up against each other and cause tiny abrasions, similar to light scratches. Those little abrasions may not affect how your hair looks or feels but they're always there. If you rub and pull your hair and then follow up with a comb or a brush, you've inflicted a lot of stress on your hair at one time. Yes, there are some women who can comb and brush their hair all day without a problem, but those women usually have thick strands and very few (if any) kinks or coils. Finer strands break more easily than thicker strands, and it seems that many African American women have fine hair. In addition to that, kinky hair is inherently porous which means that kinky hair is automatically more fragile than other types. "Damage prone" can really be an understatement when describing African American hair.
But, no matter what your hair type, the longer your hair gets, the more demanding it will be. Shoulder length hair is already about 2 years old so any weaknesses in your hair care routine will begin to show, just like bad eating habits will start to catch up with you as you get older and your metabolism slows down. If you hit a plateau in your healthy hair journey, get at least ½ inch cut off by a professional, stop brushing your hair (if you've been doing that), and apply a reconstructing treatment every week or two. Whatever you do, just don't make the mistake of thinking you've avoided damaging your hair. The only way to avoid damage is not to touch your hair as it grows out of your scalp, and we all know that's impossible. If you love your hair, you want to style it and show off all of its glory. There's nothing wrong with that. You wouldn't buy a beautiful new sweater and leave it on the shelf all the time. What fun would that be? Instead, you have to wear it lightly, wash it gently, and get small holes mended as soon as they appear.