The ability to take regular exercise becomes impossible, so your body loses out on valuable endorphins (as well as muscle tone). Pain robs you of your sex life; your self esteem; your patience; your spontaneity; your ability and pleasure to do the most basic of things; and very often your spirit.
If you haven't already heard of the 'Spoon Theory', I suggest you have a quick read. It is a brilliant way to help those around you to understand just how it is.
In brief, imagine you have 10 spoons in one hand. Each task you undertake during the day costs you one spoon. You get out of bed: one spoon. You brush your teeth: one spoon. You make breakfast. You take a shower. You walk the dog. One spoon. One spoon. One spoon. Before you know it you've run out of spoons.
Learning to pace yourself if you suffer from a chronic illness is one of the hardest and probably one of the most valuable lessons you can learn. And while it is hard enough to learn, it is even harder to teach those around you. Share the Spoon Theory with them. It might not save your life, but it will save you some spoons. :)