When a dog licks it releases pleasurable endorphins, which gives dogs a feeling of comfort and also relieves stress.
Licking can be a sign of affection – if excessive and you don’t want the dog to ‘kiss’ you then ignoring it and not giving the dog attention can decrease it.
Dogs can lick themselves out of boredom, stress or anxiety, in which case you need to give them the physical and mental stimulation they need, and address the stress or anxiety.
Licking can also be caused by allergies, such as to plants, insects, food or medication. If you think this may be the case take your dog to the vet - they will be able to give you advice and run some tests.
Dogs may also lick because they like the taste of an owner's skin.
Pay close attention to the licking of their bums, because excessive anal cleaning can indicate that the glands need to be expressed/relieved.
Dog saliva has healing properties – it contains enzymes that kill off bacteria, so when a dog licks himself, it may be to get rid of dead tissue and clean dirt from wounds.
If your dog licks you there may be a hidden message, e.g. the water bowl is empty or she is hungry. Be sure to consider the possibility that something is making her physically uncomfortable. For instance, if you notice your dog biting her paw repeatedly, she could have a thorn or sharp stone stuck in her foot pad. Treatment
For skin allergies: Changing the dog’s food, changing the shampoo or getting salmon oil to mix in the dog’s food.
For anxiety or stress: Adaptil for dogs - it’s a plug-in and it infuses the air with calming oils.
For hormonal imbalance: Phytopet Balance - a liquid herbal supplement that contains a combination of botanical extracts to help support hormonal balance.
If a gastric foreign body is found, surgery may be required.
If a bacterial infection is found, antibacterial medications may be needed.
(Source for some of the above: Why Dogs Lick, Dogs that Lick, Ask Victoria Stilwell