The Simcha of a Siyum

The Mishna at the end of Taanis (26b) quotes Rabban Shimon Ben Gamliel who declares that the most joyous holidays on the Jewish calendar are Yom Kippur and Tu B’Av. The Gemara states that Yom Kippur was joyous because the nation was forgiven for its sins. However, the Gemara wonders what was so special about Tu B’Av.
A variety of reasons are offered by various Amoraim. The Rashbam (Bava Basra 121a) says that the Amoraim are not arguing with each other. Rather, each one is simply declaring the reason that he heard from his Rebbe.
Rabba and Rav Yosef (Taanis 31a) explain that on Tu B’Av they stopped cutting wood for the altar. From Tu B’Av and onward, the summer heat is less intense. Hence, any wood gathered from then on would be more moist and not optimal for burning. Furthermore, moist wood may become wormy. Wormy wood may not be placed on the mizbeach. It is similar to offering a blemished sacrifice. Some suggest burning wormy wood on the altar is akin to offering a non-kosher creature as a sacrifice.