Answer: To rejoice over the friend’s joy is to feel happy that we reach our spiritual goal together, that we are on the path together. It’s when I feel the friends’ yearning within me and don’t feel my own presence or personal eagerness toward the goal, but rather how I aspire to it with them. In fact, my “self” disappears, dissolves into the others. This is how I attain eternal life, too.
A person’s “self” no longer exists. It functions only as a conduit of the Creator’s Light to all others. So it turns out that all our “selves” disappear. And instead of them, there emerges one integral whole, multiplied by the number of the “selves” that there used to be.
From Lesson 1 in Moscow 1/14/2011, “Introduction of the Book of Zohar”