http://revach.net/article.php?id=2525

Section: Parshas HaShavua   Category: Medrash
Medrash Matos: Sticking Together Reuven and Gad Stay Clean, But Pay The Price
Reuven and Gad asked Moshe to give them the land across the Yarden instead of in Eretz Yisroel proper.  The Medrash HaGadol says that they asked for this purely for reasons of Tzidkus.  Since they had such an abundance of cattle and livestock they were afraid if they were in Eretz Yisroel they would not be able to control their sheep from grazing in other people’s fields.  To avoid the issue of Gezel they wanted to be separated and live in a territory that would be self-sustaining.

Reuven and Gad were driven out of their land and into Galus before all the other shevatim, says the Medrash, because they separated themselves from the rest of Klal Yisroel.  Rav Yehuda says we see from here the terrible effects of Machlokes (in this case literally separatism).   Their reason for separating was totally L’Shem Shamayim and to prevent machlokes, yet the mere fact that they separated deserved a punishment.  The Medrash concludes, if they were punished, Kal V’Chomer someone who separates because of hatred and competitiveness.