The Gemara (Taanit 30b-31a) offers six reasons why Tu B'Av is a joyous day. Of those six reasons, here's the one which speaks most to me this year: when the Israelites were wandering in the wilderness for forty years, 15 Av was the day on which those who were destined to die in the wilderness finished dying. On this date, the intimacy with which the Holy One of Blessings spoke with Moshe was restored, and as a result, blessing flowed to the entire nation.
I can't speak to the historical veracity of that story, but on a spiritual level I find it very moving. It tells me that the relationship between God and the people was so badly strained by the incident with the scouts (where the scouts went into the Promised Land but returned with a fearful verdict instead of trusting in God), and by God's response to that incident (declaring that this whole generation which had known slavery would perish in the wilderness), that until the karmic consequences were complete, they couldn't really connect to each other.
It's like what happens when a child misbehaves, and the parent gets angry and declares a punishment, and then both parties feel distant from each other until the punishment is complete. A parent grounds a teenager, and during the period of the grounding, their connection isn't quite working. The parent knows the grounding was the right thing to do, but once the initial anger wears off, the parent suffers from the distance between them, too. And then the period of being grounded is over, and they can relate to each other in a different way again.
The "grounded" metaphor may seem a bit frivolous, but there's also a connection between this day and the actual ground. Here's a midrash:
R. Levi said: On every eve of the 9th of Av (during the 40 years when the Israelites wandered in the wilderness) Moses used to send a herald through the camp and announce: Go out to dig graves. They would go out and dig graves and sleep in them. In the morning he would send a herald and say: Separate the dead from the living.” They would arise and find their number diminished. In the last of the forty years, they did this but found themselves undiminished. They said; we must have made a mistake in counting. They did the same thing on the tenth, eleventh, twelfth, thirteenth, fourteenth, and fifteenth, but still no one died. When the moon was full, they said; it seems that the Holy One has annulled the decree from all of us, so they made the fifteenth a holiday. —Lamentations Rabbah, Prologue 13
This midrash takes place while the Israelites were wandering in the wilderness. Each year, the people dug themselves graves and lay down in the graves, expecting to die. One year, they stayed there until the full moon, whereupon they realized that if God had not yet taken them, the "plague" of being condemned to die in the wilderness must be over and God must have forgiven them for the misdeeds of the scouts. So on this date, they emerged from the earth -- literally rising up from the ground in which they had lain all week. Today is a day of great rejoicing because it's the day on which our people realized we had been wholly forgiven.
On this date, says the Talmud, the last members of the generation who had known slavery died. The last connection to slavery and to constriction was released. And the punishment which God had declared was finished. As a result, God was newly able to speak with Moshe intimately again, and to pour blessings through Moshe into the whole people. I imagine God saying, "That was no fun for either one of us; I'm so glad that's over; let me shower you with love from now on!"
On this Tu B'Av, may we all find ourselves able to shed the last vestiges of whatever has been constricting us -- and may we find a new way to relate to God and to each other, steeped in abundant blessings and love.
For more on this: Happy Tu B'Av, 2011, a post which explores the custom of dancing in the vineyards in white dresses, Rabbi Jill Hammer's teaching that this day is a hinge between harvest and fruition, the feminist undertones of a sex-positive Jewish festival, and a teaching from Michael Strassfeld.
Image borrowed from Tu B'Av: the Love Connection.