This is very unlikely to be a habit. It is far more likely that their feet are itchy and they are chewing/licking to try and get some relief. Unfortunately just like scratching a mosquito bite it is fulfilling at the time but doesn't help the itch to go away, often worsening the itch and causing tissue damage and secondary infection. Many dogs that do this have dried saliva staining their feet brown which can be one telltale sign. Many dogs with itchy feet (Pododermatitis) are also prone to ear itching and infection.
If you notice your dog licking its feet more than a few times a day, the skin under toes and webbing of the foot is reddened or the feet are stained, it should be investigated thoroughly in a consultation to try to achieve a diagnosis. Several different causes may include Contact Allergy, Atopic Dermatitis, Food Allergy, Demodex Mites or Bacterial/ Fungal Pododermatitis. Once diagnosed the appropriate treatment can be started and hopefully reduce or fix this painful condition which unfortunately often goes undiagnosed and untreated.