A Siyum to Arouse Brotherly Love

Sefer Siach Sarfei Kodesh also quotes the Chidushei Harim zy”a as explaining that the reason for the custom of making a siyum during the Nine Days is because the Bais Hamikdosh was destroyed because of sinas chinam (Yoma 9B) and sharing in a seudah for the honor of Torah can rectify this sin by increasing brotherly love. This concept is seen from the Gemara (Shabbos 118B), which relates that Abaya said that when a young scholar completed a Masechta, he made a festive holiday for the Rabbanan. Creating such unity through Torah is a means to fix the sin of senseless hatred.