What is the difference between a rabbi and a Rebbe?
There are many answers, here is one:
A rabbi answers questions, a Rebbe answers people. A rabbi hears what you say with your mouth, a Rebbe hears what you are saying with your soul.
I'll explain. A teenage boy once asked the Rebbe, "Do we believe in reincarnation?" The Rebbe's answer was short and cryptic: "Yes we do believe in reincarnation. But don't wait until then."
Seems a puzzling response. The boy asked a simple question, which could be answered with a yes or no. What did the Rebbe mean by "don't wait until then"?
The Rebbe was responding to something more than the technical question. The Rebbe knew how to answer the person, not the question. Most of us respond to a question posed to us. The Rebbe would respond to the person behind the question.
When this boy asked about reincarnation, he was not asking about the abstract theological concept. He wanted to know if this lifetime is all there is, or if there is more. The possibility of reincarnation changes the way we look at life. We were here before, and so some of the events that happen to us now may be leftovers from a previous life. And we may live again, which means that we get another chance to complete unfinished business from this lifetime in the next.
This is what the Rebbe was warning the young boy. Reincarnation doesn't mean procrastination. Don't use it as an excuse to put off to your next life what you need to achieve in this life.
This is the power of a Rebbe. A regular rabbi, when asked such a question, would start quoting mystical sources and explaining complex doctrines. But the Rebbe, in this brief exchange with a teenage boy, taught a practical and comprehensive worldview.
We may have had past lives, and we may have future lives, but don't wait until then. Do the mitzvah today. Do it now.