system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some fabrics get softer when they're washed a lot, while others get rougher?
gpt
It depends on the textile. With wearing and use, some materials relax and become softer and looser because the thread is bent, pulled, and twisted through mechanical motion, much the way that crumpling a sheet of notebook paper over and over makes it very soft. Wearing the clothes often contributes more to this than the washing by itself. 

However other materials may contain oils, waxes, and other materials naturally present in the fiber such as lanolin. Washing, and especially bleaching, removes these soft, greasy, fatty substances which can make the fibers dry, scratchy, or cause them to begin to fray or lose their water proofness. The primary purpose of fabric softeners, is to help replenish these substances, but it's always a downhill battle short of soaking the clothing in lard or oil. 

Lastly, some synthetic cloth can become brittle with use, similar to the way that bending a plastic spoon back and forth in the same spot will cause it to snap. The fabric begins to get creases and folds in it that don't come out with ironing. They don't exactly get scratchy, but they become stiff like folded paper, and don't bend and take the shape of the body as well.  

As fibers wear and break and tear, it has an effect of reducing thread count. Thread count is the number one thing for comfort up close to the skin, with higher thread count cloth feeling softer and silky versus lower but thicker thread count like a knitted sweater which can feel rough.