Why should missed prayers and fasts be made up for?

Missed prayers or fasts must be made up because they are a debt upon the servant, and the debt owed to Allah has a greater right to be fulfilled. Missed fasts are to be made up, but missed prayers due to menstruation are not. Missed prayers due to an excuse, such as sleep or forgetfulness, are to be made up, based on the Prophet's (peace be upon him) saying: "Whoever forgets a prayer, let him pray it when he remembers it; there is no expiation for it except that." As for prayers intentionally abandoned, there is a difference of opinion regarding the obligation to make them up, but the majority of scholars believe they must be made up, and this is the more cautious approach.