The Simcha of a Siyum

A mishnah in Ta’anis declares that the most joyous holidays on the Jewish calendar are Yom Kippur and Tu B’Av. The Gemara says that Yom Kippur was joyous because the nation was forgiven for its sins. But what was special about Tu B’Av? Several reasons are offered. The Rashbam (Bava Basra 121a) says that these Amoraim are not arguing with each other; rather, each one is declaring the reason he heard from his rebbe.
Rabbah and Rav Yosef explain that on Tu B’Av, they stopped cutting wood for the altar, because afterwards, the summer heat is less intense and any wood gathered would retain its moisture and not be optimal for burning. Furthermore, moist wood may become wormy, which may not be placed on the Mizbei’ach, as it would be like offering a blemished sacrifice.
It is not immediately clear what Rabbah and Rav Yosef’s intent was. Why is the fact that they could no longer cut wood a reason for celebration? Just the opposite – there is a mitzvah they are no longer able to perform, so they should be sad!