Why did the three tribes that comprised Degel Machaneh Reuven (Reuven, Shimon and Gad), encamp specifically in the south?
Ramban (in Pasuk 2): Hashem said to Moshe: 'Reuven, the Ba'al Teshuvah, 1 shall encamp in the south, from where the dew and the rain of B'rachah emanate - because Teshuvah is a good Midah which causes Hashem's Mercy to descend upon the world; and next to him is Gad, the mighty one, 2 with Shimon (the sinner) in between them, so that they will atone for him. 3 They shall travel second!' 4
See Rashi, Vayeishev, Bereishis 37:29.
See ve'Zos ha'Berachah, Devarim 33:20.
To shield him from Ya'akov's curse (R. Chavel's footnotes).
Ramban: 'Because after Torah comes Teshuvah'.
