The Talmud (Bab. Ta'anith 26b) tells us that the 15 of Av was a fantastically happy time celebrated with joyous festivals. Why?
One reason given is that the traveling Israelites who left Egypt stopped dying. Since we have a tradition that none of those who left, except for Kalev and Yehoshua', were permitted to enter the land, presumably this is because the last person from that generation finally died. Also, since there were certainly children who left, and since they wandered the wilderness for 40 years, this also tells us that youngest person of that generation died at younger than 40 years (if the person was a newborn when they left, and the 40 years of wandering hadn't ended yet). This does not sound like a great thing to celebrate.
Another reason given is that the tribes were given permission to intermarry. This had to do with land ownership and tribal autonomy. Presumably, then, the land had been fully settled at this point, which allowed the tribes to intermarry without worrying that any land disputes would arise. However, we know that parts of the land never did get properly settled (eg., Eretz HaPelishtim), and even if they had been, this shows that the new immigrants couldn't trust each other. This also does not seem like grounds for celebration.
A third reason given is that the tribe of Binyamin was re-integrated after a bitter civil war. OK, that's good. But their numbers were decimated, and this was really just a concession that they had gone out of bounds and were accepting the terms of their surrender and shame.
Removing roadblocks to Yerushalayim and burying the dead of Beitar, two other reasons given, seem to also be examples of the end of a tragedy, and the final reason given, the conclusion of preparation of the wood (on an annual basis) for the firewood for the Altar, seems like the only event that might warrant an institutional celebration, but it's one that might be done in the city and the confines of the Beith HaMikdash, not necessarily throughout the land.
So what's the real* reason this day is celebrated so joyously?
*Perhaps "underlying" is a better word to use here. Several different versions of this question are floating around in my head; one version wonders if there is some historical or cultural significance to this date that might be explained Aggadically.