In 20:12 in the middle of the Ten Commandments we encounter the commandment to honor one's parents. The passuk then adds, "למען יאריכון ימיך על האדמה" – so that you should extend your life on the land.
There is a debate among the commentators whether this is one promise, that you will achieve longevity in the land, or two promises, that you will achieve longevity and will have Eretz Yisrael to live in.
What does this have to do with honoring one's parents? Why is this reward promised here rather than by any of the other commandments? What is unique about honoring one's parents above all the other commandments that this one gets a specific promise of reward while the others do not? And why is the reward longevity of all things?
My thoughts are that this is because honoring one's parents is such an integral commandment and is really the basis of almost everything that it deserves extra, to show its importance. By honoring your parents you are showing the importance of tradition, which is what parents teach the children, and your connection to your past and to Hashem.
Naturally, that will also lead to longevity, and longevity in Eretz Yisrael. A person who honors his parents shows his connection to tradition and his dedication to continuing that tradition. Such a person will ensure that, aside from him continuing the tradition, he makes sure the tradition continues in his future generations. That will give his life longevity, as his life will be extended through his children.