Q: What is the reason annexed to the fifth commandment?
A: The reason annexed to the fifth commandment, is a promise of long life and prosperity (as far as it shall serve for God's glory and their own good) to all such as keep this commandment.

Ephesians 6:2-3. Honour thy father and mother; (which is the first commandment with promise;) That it may be well with thee, and thou mayest live long on the earth.

Westminster Project - John Flavel's explanation of the shorter catechism on the fifth commandment