In this chapter, the descendents of Reuben and the descendents of Gad decide that they like the land on this side of the Jordan. Making a deal with Moses, they will support their "brothers" in taking their own land, and receive as their share of the inheritance the land to the East of the Jordan. This “story” is used to explain why the Israelites are spread out on both sides of the Jordan.  It is also used to demonstrate how they strongly they felt as one people, with descendents of Reuben and Gad willing to go to war to help their brothers.  

Question: Do you think that the decision to stay on the far side of the Jordan reflects a lack of faith in their eventual victory in the land of Canaan?  
-or-
Was it simply a matter of the land being suitable for their flocks?