As it is well known, the Ten Commandments are separated into two general categories. The first five commandments describe laws about the relationship between man and Hashem, and the last five commandments describe laws about the relationship between man and man. This rule is true for all except one; the commandment to honor one's father and mother is included in the first group. Why is this commandment in the first group if the first group deals with the relationship between man and Hashem, and the commandment to honor one's parents focuses on a relationship between man and man?
The Gemara in Kiddushin (דף ל:) provides the following explanation, stating that whoever honors one's parents honors Hashem. Therefore, the law of honoring one's parents becomes a law pertaining to one's relationship with Hashem. Still, we may ask why this is so. One answer is that those who honor their parents indicate both a willingness to accept authority and to carry on the Jewish tradition. On the other hand, those who reject their parents' authority will tend to reject Hashem's authority and will not continue the chain of tradition.
This is also why the Torah states that whoever honors his parents will prolong his days (שמות כ':י"ב). This refers to both one's own individual life and also to the endurance of the Jewish people. Whoever exhibits respect towards his parents will receive respect from his own children. If one does not respect his own parents, his own children will not respect him. The parable is told about a small boy who once saw his mother drop a dish. "You clumsy idiot!" the boy yelled. "Can't you do anything right?" The boy's father heard this outburst and rushed over to the boy to reprimand him. "What's wrong with you, is that any way to talk to your mother?" The boy looked at his father and replied, "Well, that's the way Mom yells at Grandma when she drops a dish." In this situation, disrespect for one's parent had obviously been handed down from one generation to another. On the other hand, of course, whoever learns the basis of Jewish tradition from his parents will transfer that knowledge to his children and to the entire community, and thereby perpetuate the never- ending linkage of the Jewish generations. For this reason, the Gemara (שם) says that whenever a man honors his parents, it is as if he has brought down the Divine Presence to dwell with him and has honored Hashem Himself.