As you age, your teeth get worn down naturally, and the soft tissue around the mouth descends. This can leave you with a Richard Nixon smile - showing all lower teeth and no upper teeth. Teeth are going to move until they touch something that will make them stop. That might be a tongue, dental appliance, or other teeth. As teeth wear down and get shorter, this changes the normal tooth/gum ratio and gives the "gummy smile."
Find out more about this book:YOU: Being Beautiful: The Owner's Manual to Inner and Outer Beauty