If your dog is licking you all the time, most likely it is to show affection. Additional reasons may include something as simple as food residue leftover from lunch or something more complex such as a bad habit.
Dogs lick people for a variety of reasons. The most common reason is to show affection, but several other factors may be the leading cause of your dogs tongue lashing.
The most simple reason is that you just may taste good. If you have food residue on your hands or your skin, your dog' sensitive sinuses will make it hard for him to resist the urge to try and sneak a taste. After a long workout, the salt on your skin from sweat is also a hard temptation for your dog to resist.
Many times a dog uses licking as a way to gain attention. The dog also could develop a compulsive disorder where the licking becomes a habit that is hard to break.
Lastly, it may surprise you to know that your dog may be licking you in order to help heal a wound that you have exposed on your body. The saliva in the dogs tongue contains bacteria fighting enzymes that encourage healing.