There may be many reasons why dogs bite people’s feet. Dogs bite things to explore their surroundings. When a dog is curious about your feet, he may bite them to explore the taste and feel of you. If a dog feels scared or unsafe, he may bite the person's feet to express his uneasiness.
Dogs sometimes bite people's feet to play with you. This is a form of social interaction, and your dog may want you to play with it. A dog may bite a person's foot because he or she wants your attention. If you don't give your dog enough attention, he may use this to get your attention. In short, if your dog likes to bite people's feet, you may consider providing it with enough exercise and stimulation so that it can burn off excess energy, and also pay attention to giving it sufficient attention and caressing.