Chewing gum loses its flavor in time because the sweeteners are washed away by saliva in the mouth. The components of the sweeteners begin breaking down eventually.
Chewing gums are made from a base, coloring, softeners and flavoring. The base of the gum is what allows the gum to be chewed for so long. It does not break apart while being chewed as most foods do. The softener helps the gum stay soft instead of becoming too hard to chew after an extended period of time. The sweetener and flavors are added to make the gum enjoyable to chew. As the gum is chewed, the mouth fills with saliva. As the gum chewer swallows, the flavor and sweetener are swallowed. Gum loses its flavor once all the sweetener has been digested.