There is no single answer to why dogs lick their paws, but 90% of the time, it's because they itch. It's a complex problem with no easy solution. But the more you know about it, the better you will be able to help your dog.
The skin in a dog's paw is super-sensitive to inflammation. That's why dog skin allergies almost always start in their paws. Other sensitive areas include their lips and muzzle, their legs, beneath their eyes, their tummies, armpits, genitals and rectum. Often the itching starts in their paws and then spreads to one or more of the other sensitive skin areas. Additional symptoms will include ear infections, runny eyes, carpet scooting, diarrhea, and a dry hacking cough that starts around 9pm and can last until morning. All these symptoms are caused by inflammation.
Things like yeast infections, bacterial infections, skin cracking, hair loss, etc. are secondary effects of the inflammation. When the skin is compromised by inflammation, and it itches, the constant licking can disrupt the normal ecosystem on the surface of the skin, which can result in a runaway infection that would normally not happen if the skin was in a normal, healthy state.
Regardless of whether it's pollen, or dust mites, or mold, or dry air, or a food allergy, or just about anything else, the skin tends to react the same way, with a massive, overall inflammatory reaction. The treatments are also very similar. Whether they are steroids or antihistamines or supplements, they are usually aimed at reducing inflammation, and thus reducing the itching that goes along with it.
If you can find the right combination of things to do, you can usually make your dog a lot more comfortable no matter what is making them itch. If you can stop their itching, they will stop licking their paws.
It's virtually impossible to keep your dog from breathing in allergens. Maybe you can eliminate certain foods, or something obvious like fleas, but trying to prevent any contact with possible allergens is probably not going to be very effective. Concentrating on reducing the inflammatory response is going to be the best way to manage symptoms. Exactly what works, or what combination of things work, will depend on your dog. Their breed and the environment where you live are the two biggest determining factors in what's going on, and what you can do to help.
When someone points to paw licking as a behavioral issue, they are grasping at straws. It's an easy answer because they don't understand the issue. Dogs are susceptible to behavior modification because they want to please us, but you can't train a dog to stop itching. You are just making them more miserable if you start using training techniques to prevent them from scratching their itchy feet. It's misguided and not a very nice way to treat your best friend.
Dogs don't lick because they are bored. They do most of it at night when they are ready for bed. Dogs are happiest when they are getting ready to go to sleep and everyone is settling down for the night. They are tired and trying to get comfortable, just like you. They aren’t bored.
It's also not caused by anxiety. Most dogs will stop licking if given a corticosteroid. Usually a steroid will increase anxiety, not relieve it. Small dogs will often lick while sitting in your lap. They feel less anxiety in your lap than just about anywhere else. When a dog suffers separation anxiety they exhibit a whole range of behaviors, but licking their paws is almost never one of them. Certainly, some dogs do it more when you’re gone, but that’s just because they know you don’t like it.
There is no doubt the overwhelming reason dogs lick obsessively is because their skin itches. The reason they lick more at night is because of changes in the hormones, circulation, and body chemistry tied to their internal biological clock. If you have arthritis, or asthma, or just about any kind of pain or inflammation, you know it's much worse at night than during the day. The same is true for itching and dog paw licking. It gets worse a few hours after sunset and especially right before sleep.
Itchy skin is caused by dryness, allergies, and a hundred other things. But in general, it is an inflammatory response of the skin to whatever the cause. Inflamed skin itches, your dog licks it, things get worse. Downward spiral from there.
Althought the biggest single reason for dog paw licking is inhaled allergies, the genetics of different breeds plays a huge role. Most dogs with severe skin problems are pure breeds or cross breeds. The smaller the gene pool of a breed, the more skin problems they generally have. Dogs with white skin are horribly sensitive to everything and get secondary infections more easily. Bulldogs have toe cysts. Great Danes chew one ankle. Jack Russells all lick their paws constantly. There are about two dozen breeds that seem to have all the allergies. Very few dogs outside that group show any problems to allergies or generalized skin conditions.
Since there’s not much to do about the genetics of dog breeding, or pollen in the air, and no one has found a cure for allergies, dog owners can only rely on symptom relief and management.
Luckily, there are a number of options to help relieve their symptoms. Often it takes more than one thing to provide the greatest amount of relief.