The myth of werewolves goes back to ancient greek literature and covers most cultures across the world, from American tribes to various Asian cultures where the wolf is replaced with tigers or leopards. The symbolism and myth of the beast still prevail today and were even used by Hitler to bring fear.
The actual transformation process can be triggered by many things and can take many forms. Some myths tell of the slow transformation of man to beast, with bones reforming and skin stretching... painful to be sure.
But not as painful as the beast bloodily ripping itself from within the man, leaving ragged tatters of flesh clinging to his fur. You can see why TV programs shy away from this version of the transformation.