you might think of tight muscles as being "addicted" to contraction. You can send the relax message all you want, but try as they might, they cannot return to their normal resting length on their own.
In fact, the longer your muscles remain overly tight, the worse things get. One of the reasons is your body thinks it's doing you a big favor by keeping the muscle just under the length that would cause it stress (the normal resting length -- which does stress it due to the muscle's new habit of being too short).
If you notice this situation early on, you may be able to remedy it by doing some specific stretching or movement therapy. After a while, though, little tiny adhesions will form. They'll respond better to mechanical therapy (massage therapy is one example) than stretching or movement.
Eventually, if you continue to leave a tight muscle situation unaddressed, your bones will re-shape themselves in response to the force your body has applied. These skeletal system changes are much more challenging to un-do, as you might imagine.
Henry hubbard therapy