“כֹּהֲנֶיךָ יִלְבְּשׁוּ-צֶדֶק; וַחֲסִידֶיךָ יְרַנֵּנוּ.”
“Let your priests be clothed with righteousness and your pious people (Chassidim) be filled with joy”
The priests draw the Light of Chokmah from the Left column. The Levites are from the Left column and serve the priest to continue the Light of Chokmah to this world, which is the aspect of bringing joy. They are doing their work with joy, happiness, and sing when serving in the Holy Temple. In this verse King David skips the Levites and assigns the work of bringing joy to the pious people (Chassidim, Chessed) that are the aspect of the Right column.
“לֹא-הֶחֱזִיק לָעַד אַפּוֹ, כִּי-חָפֵץ חֶסֶד הוּא”
“He does not retain His anger forever, Because He delights in unchanging love (Chessed).”
King David knew that God’s attribute of Chessed will accept his invitation as is and he continued asking;
“בַּעֲבוּר, דָּוִד עַבְדֶּךָ– אַל-תָּשֵׁב, פְּנֵי מְשִׁיחֶךָ.”
“For the sake of David Your servant, Do not turn away the face of Your anointed.”
“נִשְׁבַּע-יְהוָה, לְדָוִד אֱמֶת– לֹא-יָשׁוּב מִמֶּנָּה:”
“YHVH has sworn to David A truth from which He will not turn back:”
The Holy One Blessed be He accepted King David’s special invitation and the pious people (Chassidim) became channels for the Light of Chokmah on the Left.
The Zohar tells us that from King David’s invitation we learn that in a person’s home one can act as he pleases but if one is invited to another place then he must follow the ways of the host.
The big picture revealed to us is that we are guests in this body and in this world. We should follow the ways/system of the host, Who is the Creator Blessed be He.
When we build a proper vessel to host a greater Light, using the tools given to us through the Torah, we can invite the Creator to fill our lives with Chassadim.