This story is related by Rabbi Yosef Yitzchak of Lubavitch and can be found on Chabad.org
A Chasid, Reb Yosef Yuzik once asked his Rebbe, "Rebbe, what is a Chassid?"
Replied the Rebbe: "A Chassid is a lamplighter. The lamplighter walks the streets carrying a flame at the end of a pole. He knows that the flame is not his. And he goes from lamp to lamp to set them alight."
Asked Reb Yosef Yuzik: "What if the lamp is in a desert?"
"Then one must go and light it," said the Rebbe. "And when one lights a lamp in a desert, the desolation of the desert becomes visible. The barren wilderness will then be ashamed before the burning lamp."
Continued the Chassid: "What if the lamp is at sea?"
"Then one must undress, dive into the sea, and go light the lamp."
"And this is a Chassid?" Reb Yosef Yuzik asked.
For a long while the Rebbe thought. Then he said: "Yes, this is a Chassid."
"But Rebbe, I do not see the lamps!"
Answered the Rebbe: "That is because you are not a lamplighter."
"How does one become a lamplighter?"
"First, you must reject the evil within yourself. Start with yourself: cleanse yourself, refine yourself, and you will see the lamp within your fellow. When a person is himself coarse, G-d forbid, he sees coarseness; when a person is himself refined, he sees the refinement in others."
Reb Yosef Yuzik then asked: "Is one to grab the other by the throat?"
Replied the Rebbe: "By the throat, no; by the lapels, yes."