לזכות יהודי השרוי במצוקה הרוצה בעילום שמו!!
לזכות משה יהודה בן פעשא דינה
לזכות ר' יהודה יעקב בן דינה חאשע ואשתו היקרה
This is deep.
Beyond what I can explain.
A surface explanation:
Simcha is especially necessary at a time of spiritual falling. At that time, there is no defense against deep falls and fear from spiritually harmful forces, other than simcha. A simcha not only of the heart but one that is expressed in a very external way. When a person is on a high, such great expressions of simcha are not necessary because then pleasure and splendor fills all of his limbs. But when he is down it is the right time to externally display his simcha.
Similar to this is the simcha of Chosson and Kallah when a man descends from the Kabbalstic plateau of what a male represents as the gemara alludes to when saying that a man should descend from his level and marry a woman. The fear is that man will sink to depths of depravity in his lust for women. Therefore external exhibitions of simcha are necessary. Through this, the kabbalistic "gvuros" level of women will merge with the kabbalistic "chesed" level of men, the name of Hashem will be complete and the Shechina will dwell among them.
So we dance at weddings to save the chosson from a potential fall.