|Tractate Ta'anit goes on to list five different reasons for rejoicing on Tu b'Av:
During their forty year desert, female orphans among the Israelites who had no brothers could marry only within their tribe, in order to prevent their father's inherited land in the Land of Israel from passing on to other tribes. On the fifteenth of Av of the fortieth year, this ban was lifted. This we learn from the story of the five daughters of Tzelaphchad
Also during the fortieth year in the desert, the last of the generation of the sin of the spies, which had been forbidden to enter the Promised Land, found that they were not destined to die. For forty years, every Tisha B'av night, the Jews made graves for themselves which they slept on Tisha B'av; every year a number of them died. In the 40th year, the fifteen thousand remaining survivors from the generation of he spies went to sleep in their graves and all woke up the next day. Thinking they made a mistake calculaing the date, they returned each night to their graves, until the night of Tu B'Av. This being the full moon, they understood that they had not been mistaken: The decree against the generation had been rescinded!
The Tribe of Benjamin was allowed to intermarry with the other tribes. (A ban had been enacted against intermarriage following the incident of the Concubine of Gibeah, see Judges 19-21.)
Cutting of the wood for the main altar in the Temple was completed for the year: One of the early signs of the changing seasons was an increase in humidity, creating conditions in which the wood collected after the fifteenth could be wet or wormy.
More than a year after the destruction of the Jewish stronghold of Beitar and the slaughter of the entire Jewish population of the city by the Roman invaders, Rome at last permitted burial of the victims. Miraculously, the bodies had not decomposed, despite their being left unattended for the entire year. This was a sign of G-d's mercy.
During the time of the second Holy Temple, the entire seven day period, beginning with the 9th of Av an concluding with Tu b'Av, was a festive holiday celebrated in the holy Temple. When the Temple is rebuild and the Divine service renewed, so too will the seven day festival of Av be once again celebrated by Israel!