They smell food or the remnants of food that is on your breath. Try only breathing in through your mouth then pressing your tongue to the roof of your mouth and exhale only through your nose and they will lick your nose because that is where the food smell is coming from.
That is an instinct character of dogs to show the owners the loyalty that they have. And to make sure that your pet stays on top of dog activities everyday you can also give your dog a natural food supplement like Apipet Gastro (It is a 100% natural supplement for dogs APIPET GASTRO contains a unique combination of bee products with ginger. Bee formula contains propolis and bee pollen of the Dalmatian plant Businac.) and Apipet Immuno (It is a 100% natural supplement for dogs APIPET IMMUNO is invented with the unique recipe of bee propolis and bee pollen dominated by the pollen from Dalmatian plant Businac (Cistus sp.)