Of all the myriad spiritual worlds, the physical world is unique. It is the only world where completely disparate spiritual entities can rub shoulders just by living in physical proximity. The most extreme case of this anomaly is our body, in which many different emotions and thoughts are forced to either cooperate or struggle for survival. This struggle normally lasts a lifetime, leaving a battlefield strewn with misery, distress and strife, after which the body decomposes and the various facets of the soul each go their own way.
The life of a Tzaddik, however, is different. He spends his life integrating all the various forces within him, using each one in turn in the right time and the right place, creating a symphony of emotions and intellect that bless his surroundings with ethereal music. When he dies, the various facets he integrated do not separate. He continues to exist just as he was before, only without the need for a physical body to pull the pieces together.
For seventeen years Yaakov Avinu enjoyed a life that was close to the ultimate perfection one can attain in this world. His sons were all united physically and spiritually in serving the Almighty, with Yosef administering to all their physical needs. Seventeen years, corresponding to the value of the word 'Tov', good, to allow people to see with their eyes that the aftermath of a life of righteousness is pleasant. When these years were over, he gave his sons advice in attaining their ultimate goals, and then he prepared himself for the next world.
Unlike other people, Yaakov Avinu chose when he wanted to die. By elevating his feet to the level of the rest of his body, he was no longer susceptible to the natural cause and effect that determine people's fate in this world. He was ready to live an eternal life in the world of eternal good.
Adapted from Rabbi Vali's book on Bereshis. Questions and subscriptions can be mailed to: the Yeshiva