If you will ponder the various reasons for TU b'Av, you can see in each one a message of consolation and promise... and therefore joy.
In the aftermath of the "Sin of the Spies", G-d decreed that the entire adult male population of that generation should die out during 40 years of wandering. Not only was this decree handed down on Tish'a b'Av, but the "sentence" was carried out on Tish'a b'Av each year. Tradition tells us that each year on the 9th of Av, the men of Israel would dig graves and sleep in them. In the morning, Moshe would announce "let the living separate from the dead". Each year, approximately 15,000 men were thus buried, the others living on for at least another year.
In the final year of wandering, none of the remaining 15,000 died. Not on Tish'a b'Av eve, nor the next night, nor the next. The people thought they had erred in the calculation of the days of the month, but when the full moon of Av took its place in the sky, all joyously realized that the decree against the remaining men had been rescinded through G-d's mercy. The 15th of Av was thus marked as a day of joy. Tish'a b'Av commemorates the "Sin of the Spies"; TU b'Av marks the joy of being alive and the ability to make amends and "move on". It marks (the existence of) forgiveness, even from very serious sins.
MORE TU b'Av was the end of the season for wood-gathering for the Mizbei'ach. After this date, the power of the sun begins to diminish due to the shortening of the day and lengthening of night. Since it is essential that the wood for the fires of the Mizbei'ach be free of worms and insects, it was feared that after TUb'Av the wood might not sufficiently dry out to avoid infestation. The completion of the wood-gathering season was an occasion for special korbanot in the Beit HaMikdash, and TU b'Av was celebrated as a family festival for those involved in this special mitzva.
Note that just as 9Av and 15Av were opposite sides of the same
coin vis-a-vis the Sin of the Spies, so too are they here in that 9Av marks
destruction of the Temple; the 15th celebrates wood-gathering for the Beit
For a long period of time, due to a misunderstanding in the rulings concerning who the daughters of Zelofchad should marry, there was little or no inter-tribal marriage. After a later Sanhedrin clarified the issue, inter-tribal marriages became commonplace. This reuniting of all of Israel, was associated with TU b'Av, and the cause of great joy.
MORE The mishna referred to above, tells us that the young maidens of Jerusalem would wear borrowed white dresses, so as not to embarrass one who had no dresses... and they would dance in the vineyards... (so that eligible young men might choose their brides).
This, the two items mentioned above (daughters of Zelofchad and the tribe of Binyamin), and the fact that there have not been weddings during the Three Weeks (or at least the Nine Days), combine to make TU b'Av a special day for Shiduchim and marriages, and a truly joyous day.
MORE Following the fall of Beitar about 65 years after the destruction of the second Beit HaMikdash (on a Tish'a b'Av, non-coincidentally), the Romans did not permit the bodies of the dead Jews to be buried, as a punishment for rebelling against and resisting Rome. A few years later, a new emperor allowed burial of the bodies. Tradition records that this was on the 15th of Av. The bodies were found not to have decayed in the years that they lay unburied. The Sanhedrin at Yavne declared this a miracle and composed the fourth bracha of Birkat HaMazon - HaTov V'ha'Meitiv, to thank G-d for His kindnesses, even in darkest of times.
Notice the trend here. Just about every facet of Tish'a b'Av finds its way into the nature of TU b'AV, but in an opposite - positive sense. It is almost as if we couldn't handle Tish'a b'Av without a day of a very different nature less than a week later. TU b'Av represents in so many ways the light at the end of the tunnel and the reminder that the Geula is just a step away.