Many reasons ...if it's hot , they like your sweat . Ollie likes lotion . Both my Bushwacker and my Ollie found cancer cells . If you've just eaten BBQ ribs , it speaks for itself !
yup sometimes you taste good (we are salty) and I think it is also a form a of affection, they were licked by their mothers as pups in grooming and, as grooming is a social and affectionate behavior, they copy it and groom you too.