Unless the reason for a rule is understood, it is difficult to learn the rule, and it is still more difficult to apply it successfully in practice.
-George C. Crocker, Crocker's Principles of Procedure
It can be very tempting to give the reason "because I said so" because it's so quick and easy, but it is seldom a compelling explanation for the person who hears it. It may make a toddler follow instructions for a brief time, but rarely does it lead to lasting change (anyone who has ever tried to get a four-year-old to stop jumping on the couch or bed has experienced this). And even more importantly, it fails to teach the child anything and does not give him or her the ability to apply the same concepts to new situations.