Geulah in the Merit of Simcha
The pasuk states (Tehillim 126:2-3): “Then they will say among the nations, ‘Hashem has done great things with these. Hashem has done great things with us; we were happy.” Rav Eliezer Menachem Mendel of Lelov zy”a (quoted in Sefer Divrei Dovid) and Rav Shimon of Yaroslav zy”a (quoted in Sefer Toras Shimon) explain this to mean that after the redemption, the nations of the world will ask, “Why did Hashem do great things with these people?”
We will answer, “Hashem did great things with us because we were happy. Since we were always happy, He sent us our salvation.”