The Beis Yosef explains that the 24,000 students of Reb Akiva stopped dying on Lag B’Omer, hence Lag B’Omer is a day of Celebrating. But the Pri Chadash points out that the reason the students stopped dying was mostly because there were almost none left! Only five remained!
Reb Akiva, however, did not give up and immediately began teaching Torah to those five students. And from those five students, Torah went forth to the world and continues until today.
That is certainly a reason to celebrate!
~ Adapted from R’ Elimelech Biderman Shlita