Our Gemara on Amud Beis tell us that One reason that the 15th of Av was a holiday is that it was the day in which the Jewish people, subsequent to the generation of the wilderness, were permitted to intermarry from one tribe  to another. Why is this considered such a holiday? Shall we say that the restoration of a certain type of liberty over whom you marry is really such a cause for celebration? There aren’t enough eligible mates within your own tribe?

The  תפארת שלמה על התורה שבת נחמו explains that when people of different tribes are allowed to marry it will encourage cross fertilization and influence of different spiritual potentialities within each tribe. This is no small thing. In fact, העמק דבר בראשית י״א:ו understands this to be the key sin of the Tower of Babel. God saw how they were able to organize and control people around one tower and one central vision. It eventually would lead to tyranny and oppression by not allowing others to have different opinions. This is why the punishment fit the crime. Creating different languages necessarily forced a dispersion and broadening of human experience.

I will add, the idea of taking in different qualities and blending together is the essence of shlom bayis NOT forcing people to conform and be similar. Thus, the great celebration was about understanding and allowing for diversity of opinion in order to create people who are more whole and more balanced.