Stretching is a totally natural conditioning exercise that improves the tone and health of muscles; limbers up tendons, ligaments, and joints; changes the dynamics of the nervous system; and just feels good. You can learn a lot about our need to stretch by watching dogs and cats do it. All of us tend to stretch after being in one position for a long time, and students of the human body tell us we ought to make a habit of stretching in opposite ways from the positions we spend the most time in during the day. For instance, if you work leaning over a desk, when you get home you should spend a few minutes with your head, neck, and shoulders arched backward.
Muscles contain stretch receptors, special groups of cells that inform the central nervous system about their state of tension. This may be why stretching can change our level of arousal and mood. Stretching feels good, but it is a sensation that borders on both pleasure and pain. Although some kinds of formal stretching exercises, like yoga, may seem painful when you first try them out, your perception of the sensations will change fairly quickly if you practice.
Stretching is so natural that you can easily invent your own forms of it. If you feel the need to do it formally, you can find stretch classes at many fitness clubs (aerobics classes usually include some preliminary stretching, too), or you can learn from books. I strongly recommend that you stretch frequently, especially if you suffer from stiff muscles or spend a lot of time sitting or working in one position.