There are several reasons for the simcha of Lag Ba’omer:
1) On Lag Ba’omer the talmidim of Rebbi Akiva stopped dying. (Mishna Brurah 493:MB8)
2) It is the Yahrtzeit of Rebbi Shimon Bar Yochai and on this day he revealed the Zohar. (Sdei Chemed, Chida, Bnei Yissaschar, Chayei Odom, Aruch Hashulchan, Kaf Hachaim)
On Lag Ba’omer Rebbi Akiva gave Semicha to his five talmidim who did not die, and through whom Torah was disseminated, (among them Rebbi Shimon Bar Yochai).
(Sdei Chemed, Chida, Kaf Hachaim, Pri Chodosh)
4) On this day, the monn began to fall for Klal Yisroel in the Midbar (S’U Chasam Sofer YD 233).