Alluding to the story in Judges 21, after the tribe of Benjamin had a tragedy that almost wiped them away, the Lord allowed marriage between the tribes in order bring back to life the tribe of Benjamin. The Talmud explains that the “daughters of Jerusalem would go dance in the vineyards” on the 15th of Av, and “whoever did not have a wife would go there” to find himself a bride. After the destruction of the temple, this was not commerated as it was before the desturction of the temple. However, the day was re-introduced as the day of Love and Rebirth around 1948.
While the 9th was about destruction, the 15th of Av (the day of the full moon) is about God's Love and desire to rebuild us back unto Himself! Tu B'Av became Israel's National holiday of Love