Sos Asis

שׂוֹשׂ אָשִׂישׂ בַּה', תָּגֵל נַפְשִׁי בֵּאלֺקַי, כִּי הִלְבִּישַׁנִי בִּגְדֵי-יֶשַׁע מְעִיל צְדָקָה יְעָטָנִי, כֶּחָתָן יְכַהֵן פְּאֵר, וְכַכַּלָּה תַּעְדֶּה כֵלֶיהָ

"I will rejoice intensely with Hashem, my soul will exult with my G-d, for He has dressed me in clothing of salvation, in a robe of righteousness has he cloaked me, like a groom who axalts [himself] with splendor, like a bride who bedecks herself with her jewelry." (Yeshayahu 61:10)

Why is there a change in Hakadosh Baruch Hu's name? First it says "I will rejoice intensely with Hashem(Adon...)," and then it says "My soul will exult with my G-d(Elokai)."
So- it is a well known concept that Hashem's name is used differently in different places to pinpoint certain attributes of His. When the name "Adon..." is used it connotes Hashem's attribute of Rachamim-mercy. And when the name "Elokim" is used, it expresses Hashem's attribute of Din-Justice. That being said, Rabbi Yoseph Kimchi points out that in the first phrase the verb is in a double form, alluding to both the Guf (body) and the Neshama (soul). Both of these aspects of a human being rejoice in G-d's mercy. The second phrase, however, is in singular form, and only the Neshama is mentioned. This is because only the Neshama, which can endure far more suffering that the Guf, can appreciate the attribute of justice and rejoice in it. [It, therefore, rejoices when it is purified by Hashem's attribute of justice]
וְטַהֵר לִבֵּנוּ לְעָבְדְּךָ בֶּאֱמֶת "May Hashem purify all of our hearts so we may serve Him fully in truth!!"

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(Chuppah pic courtesy of NeedlePointShop)