You will have noticed tiny balls forming on a sheet after a while, especially at the foot end. This is called pilling. It spoils not only the look but also the feel.
The cause is friction. Fibres loosen on the surface, their ends work their way out, and as they rub they tangle together into little balls. There is an interesting distinction here. On pure cotton these balls eventually break off by themselves, because cotton fibre is relatively weak. On polyester blends, however, the polyester fibre is so strong that it keeps the ball anchored to the fabric. The balls do not break off; they build up. That is why cheap polyester-cotton blends look "worn" so quickly.
Products made from short-staple cotton also pill more; the fibre length question from the piece on long fibres comes back here. Washing items inside out, not overloading the machine and not washing them with rough surfaces all help prevent it. If pills have formed, a fabric shaver removes them without damaging the fabric.