What is the connection between man and a tree?
The unique quality of a tree, which no animal possesses, is that it is firmly and deeply rooted in its source of life and energy - the ground. And due to this firm rooting, the tree grows taller and stronger than any member of the animal kingdom.
Thus the "tree" within man is that part of his make-up which is (a) the most deep-rooted in the soul; and consequently (b) it is the most powerful. And this is: his character and emotions.
While at first glance, the intellect would appear to be a man's most expressive and "personal" faculty, chassidic thought teaches that one's emotions and character are in fact more deep-rooted in the soul. For this reason our emotions tend to be powerful and uncompromising, like a tall tree, because their deep "roots" unleash the inner wellsprings of the soul directly into the conscious arena.
Intellect, on the other hand, has no fixed roots (rather like members of the animal kingdom which are not fixed to one particular place). So we are able to be intellectually involved in all sorts of matters with which we have no personal connection, since the intellect is not so deeply rooted in the soul that it will passionately "take offense" to something which runs contrary to a person's make-up.
Likewise, changing one's mind is relatively easy, whereas changing one's personality - from miserly to generous, or from evil to good - is no easier than uprooting a tree and planting it somewhere else. Nevertheless, the Torah wishes us to do exactly that: to change our character and emotional traits for the good. In this way we bring perfection to even the innermost aspects of the soul, where the "roots" of our emotions reach.
And it is in this vein that the Talmud warns us only to study Torah from "a respectable Torah scholar" i.e. one whose knowledge "bore fruit" in the form of good deeds and fine character. For a person should seek a teacher who has both intellectual and emotional refinement, who will provide a living example of how to cultivate his "arborous" side.
Based on Likutei Sichos of the Lubavitcher Rebbe vol 24