What are the usual reasons for white spots on nails?

Can be fungal. Bright white spots on the nail, almost appearing as if you sprinkled powdered sugar on it can be a superficial fungal infection. If the nail is a normal thickness, this can be the one type of fungus that can respond to topical treatment.
Many possibilities. Some causes of nail discoloration include trauma due to tight shoes, infection, and medication side effects. You should try wearing shoes with a higher wider toe box. Get the feet inspected by a professional to determine the cause of the problem, as treatment varies depending on the diagnosis.