Answer: To unite according to the condition of mutual guarantee means to be as one man with one heart. In order to unite hearts into one, to unite all desires into one desire the way it was before the breaking of the vessels, we go through two stages.
In the first stage we neutralize the will to receive for the sake of reception and thereby carry out the condition of Hafetz Hesed, “Do not do to another what is hateful to yourself.” By virtue of this I no longer harm my neighbor. I no longer judge a person by how good or bad he is, how much I can use him, control him, and deceive him.
I no longer look for profit from the world. I simply don’t want anything. Once I attain the entire magnitude and guile of my evil, which pushes me to harm others at every second, I yearn to maximally minimize the use of my desires. I only ask for the things I cannot do without, the bare necessities, hoping that then I will attain the quality of bestowal.
I want to become a point, like Malchut that becomes included in Bina. Just one point. It’s as if I don’t exist. My “I” is included in Bina and devoted to it,like a drop of semen in a womb. For now I am just a drop that does not exist by virtue of itself, but on account of the upper one.
This is the first stage where our hearts are not united yet, but they also aren’t repelled from one another.
At the next stage they start coming closer, as it is written, “Love your neighbor as yourself.” With all of my heart (because I don’t have other instruments), I check the heart of the neighbor and see the extent to which I can fulfill him and receive from him. My heart works entirely to serve and fulfill his heart, and he treats me the same way. This is the unity of hearts that we attain by carrying out the principle of love for the neighbor as for ourselves, thereby becoming as one man with one heart.
From the 5th part of the Daily Kabbalah Lesson 7/27/11, “The Peace”