Mostly all the dogs lick on the feet. This is the way to tell that he loves you a lot. Licking feet is the most common and easiest way for the dogs to show their love and affection for you and they also expect your approval for this. It is his way of delivering the message to you. If you will resist your dog doing this or kick him away then he will think that the message of his love is not properly reached to you so he may even try harder to lick your feet the second time.
Human sweat contains salt. So when your feet sweat the salt gets collected on the skin. Dog tries to retrieve that salts because animals want to have salty products so that their body should remain hydrated. Salt lowers the moisture level that is to be released by the body. Some dogs lick because they get unintentionally trained with the belief that they can get whatever they want by licking or we can say that they start believing that their demand for getting anything they want will be fulfilled. Many times you must have stopped your dog doing this but licking had become his habit. He starts enjoying licking and believes that we human enjoys this too. It is the most perfect way for a dog to show his affection.
Sometimes when dogs lick on their own feet it shows that he might be having a scar or cyst which he wants to heel but it if he licks on both of his feet that mean it can be a sign of allergy. Veterinarian should be consulted for this.