Most of the juiciness in meat is actually water, but the sensation of juiciness happens on two fronts.

The first comes from actual moisture held in meat fibers that is released when the fibers are broken during chewing.

The secondary perception of juiciness is due to saliva flow that is stimulated by the presence of fat in the mouth. That is why a well-marbled meat will taste juicy even when it has been cooked relatively dry, while a bite of veal (too young to be thickly marbled) may start out moist and tender, but will dry out in the mouth before it’s swallowed.