Seven reasons for fasting Ta’anis Bahab.
1) Chillul Hashem & for the Heichal that was burnt . ( Ms. Sofrim 21)
2) Men & women mingle during the Drasha on Yom Tov (Tosfos Kidushin 81a)
3) Men & women overeat & mingle during the meals (Eshkol)
4) Rain for winter and no harsh spring weather so the crops won’t get damaged. (Elya Raba)
5) Change of seasons(winter & summer) shouldn’t be harmful for the people. (Levush)
6) Hashem should ease the Galus . (Machzor Vitri)
7) Zilzul Chol Hamoed (Maharam miRotenberg)