Moses told the tribes of Reuben and Gad, "I know that you have much livestock."
The tribes of Reuben and Gad wanted to remain on the east side of the Jordan River because they had an abundance of cattle, sheep, and goats, and this territory was well-suited for pasture. The reason why these tribes had more animals than the others is because they appreciated the spiritual value of the manna, and therefore made it the chief staple of their diet. The other tribes, in contrast, also ate animals from their herds and flocks.
Their appreciation of spirituality was also a direct reason why the tribes of Reuben and Gad wished to remain on the east side of the Jordan River. By opting for the shepherd's life rather than the farmer's, they hoped to have more time to commune with G-d.
Praiseworthy as their appreciation seems, these tribes missed the point. The purpose of spirituality is to refine and elevate the material world; it is our ability to make spirituality affect materiality that we should appreciate, not the abstract experience of spirituality itself.
From Kehot's Daily Wisdom