It is the only part of the Decalogue that has a reward attached – “That your days may be long upon the land which the Lord your God gives you”. The sages say that the reward is both in this world and the World to Come.
According to tradition, which places five commandments on one of the two tablets of the Law and five on the other, this one fits on the first tablet which deals with duties between man and God.
Logic, some people argue, might have dictated something different, but the tradition is well aware that even though this appears to be a commandment between man and man, it still belongs amongst the duties to God. The Talmud (Kiddushin 30b) says that if we honour parents it is as if we honoured God, the Divine Parent.
The Sefer HaChinuch (Mitzvah 33) sees this commandment as arising out of hakkarat hatov, the acknowledgement of favours received. It says that if we recognise our parents who brought us into the world and exerted themselves ceaselessly for us, it leads to acknowledging God who created us, our parents, and all beings, and protects and supports all His creatures constantly.
One more word – about the nature of the reward for obedience to this commandment. What does the text mean when it says, “That your days may be long”? Does it mean that we will live longer?
If this were the case the Torah would have used a different word – not “that your days may be long”, but “that your days may be many” (the second paragraph of the Shema uses this terminology: Deut.11:21).
It is more likely that the Ten Commandments were promising longer days in the sense that each day will give us extra quality and more opportunities of living wisely, usefully and well.