its a good question baalhaboozer, i heard once from a rebbi a few answers.
1) in the minhagei maharil it is written thatwe rejoice for the students who were spared
2) in th Biur haGr”a, the vilna goan brings a mekor from the torah that we must rejoice when suffering ends. Hashem had decreed that those over 20 yrs who left mitzrayim would die in the midbar. 40 yrs later, when the decree of death had ended, they marked the 15th of Av (Tu B’Av) as a Yom Tov.
3)the pri chadash says the celebration of lag b’omer is because that is the day r’ akiva gave smicha to his new 5 talmidim: r’ meir, r’ yosi, r’ yehuda, r’ shimon bar yochai and r’ elazar ben shamoa. it was these 5 students who disseminated torah after the tragic death of the other 24,000 students.
peace.