If we don’t realize we must consider deeply until we understand that our entire well-being is dependent upon our connection to the true tzadikim. Our connection to the tzaddik is dependent on our connection to Hashem and our connection to Hashem is dependent on our connection to the Tzaddik. For the tzaddik directs, guides, leads us to Hashem in all that he does. Our part is to become aware of the greatness of the Tzaddik and to strive to listen to and follow all that he says. This is hard for us because our understanding is lacking and much of what the Tzaddik says is beyond us. Therefore, if we want to came to attach ourself to Hashem and to true life we have to wean ourselves from relying on our own understanding and to strengthen ourselves in emunah in the tzaddik so that we listen to him in everything he says even when we don’t understand. Only in this way can we be successfully led away from our natural darkness and come close to Hashem in truth.
As far as the passing of the Tzaddik from this world; we are taught, the tzaddikim are greater in their death than in their life. The tzaddik’s power doesn’t stop when he passes from this world, rather it strengthens! Admittedly it may be harder to connect to that person you can no longer sit next to, but here, again, the important matter is emunah; even though we have little if any understanding of how the tzaddik that we are attached to is working for us where he is, nevertheless we must strengthen ourselves in emunah and pray at his grave, etc. and mostly attach ourselves to him through the words he left.