The root of Zeir Anpin’s essential nature lies in Imma’s stern judgments – her five mighty powers. In accordance with this essential nature, each light stands by itself and does not join with any other. This is because the nature of Judgment is not to display brotherly love. Rather, all is dejection and angry faces.
However, Imma overcomes this with her sweetening power. As the stern Judgment subsides, so the dejection passes and brotherly love reigns. It is for this purpose that Imma enters into Zeir Anpin and makes brotherly love reign among his Sefirot. This is done through Malchut, for Malchut was instituted as the container vessel for all of them, so that instead of each light shining separately, they are all directed to one single place.
A connection then develops between them, and they all turn to Malchut as the “hind of love” (Proverbs 5:19), and they themselves join with one another. The more they turn to her, the stronger the bond of brotherly love between them, and this brings great joy.