Cats lick humans for a variety of reasons, most commonly out of a desire to groom or bond with them. It is a sign of affection that usually does not occur unless the cat feels completely safe in its owner's presence. However, stressed cats will also suckle and lick objects that provide comfort, including human skin.
Kittens and older cats are often seen grooming each other. They usually focus on hard to reach areas, such as the ears or head. Grooming helps cats to relax and exchange scents with each other, cementing their acceptance into the colony.
When cats are comfortable with humans, they lick human hands to indicate and reinforce their feelings. However, when a cat is anxious, it may compulsively lick its owner's hands in order to soothe itself. This behavior can be deterred by distracting the cat, perhaps through the use of interactive play that encourages the cat to be active and strengthens the bond between pet and human.
Like humans, cats feel more relaxed and content after exercising. Redirecting the cat's attention is also an effective method of curbing excessive licking. Offering a toy often turns a cat's attention to an object that's more acceptable to lick.