וגשו אליו ויאמרו גדרת צאן נבנה למקננו פה וערים לטפנו וגו’ בנו-לכם ערים לטפכם וגדרת לצאנכם וגו’
[The tribes of Reuven and Gad] approached [Moshe] and said: “We will build shelters here for our flock and cities for our children”…[Moshe responded: “Build for yourselves cities for your children, and shelters for your flock”…
After the Jews conquered the land of Sichon and Og, on the east side of the Jordan River, they were prepared to enter the Promised Land. The tribes of Reuven and Gad noticed that the area they had just conquered was excellent grazing land. Being that they had ample flock to feed, they thought it would be a good idea for their apportioned land to be given from this one, instead of the land of Israel proper. They approached Moshe and told him if they received this conquered land, they would use it to build shelters for their flock, and cities for their children.
Chazal point out that Moshe subtly rebuked them by responding that they should build cities for their children, and shelters for their flock. From the fact that these tribes mentioned their flocks’ needs first, it showed that was their main concern. חיבבו את ממונם יותר מן הנפשות, they cherished, and were thus more concerned for, their money than their families. Moshe told them that their priorities were incorrect. They should be more concerned for a place for their families to live, and only as an afterthought be concerned for their flock. It’s very hard to understand these words according to their simple reading. How could it be that the tribes of Reuven and Gad had such incorrect values? They had just spent forty years basking in the Divine presence, exclusively engaged in Torah and mitzvos. They should have realized that money is fleeting and only a means, not an end. We see even the simplest of people realize that their family is more precious than their money. How can we understand this teaching of Chazal?
If we analyze the actual events that led up to this conquest, we’ll realize that the tribes of Reuven and Gad had logical point. Their main task after settling in the land of Sichon and Og was to build shelters for their flock. There was actually barely any work necessary to build cities for their families. The Torah tell us that Sichon and Og’s entire army came out of their cities to attack the Jews in the wilderness, and the Jews were victorious. Hashem’s intent was to spare the Jews the strain of waging war against every city in their conquest. This way, all their enemies came to them. As well, when they reached their enemies’ cities, they were completely abandoned, and free for the taking.
What this means is after the battle, the Jews had readymade cities available for their personal use. They didn’t need to build new cities. So the main task ahead of them was to build shelters for their flock. Therefore, the tribes of Reuven and Gad, in their mind, spoke appropriately when they preceded to mention their flock before their families. There wasn’t much work left to do for their families, so they mentioned them after saying that they would build shelters for their flock. How then is there room for criticism to them by Chazal, and rebuke from Moshe?
Moshe was pointing out to them that they were making a fundamental mistake. They though that they didn’t need to expend any effort building a city for their families. However, they didn’t take into account from whom they inherited these cities. They were some of the worst, immoral idol worshippers. Their houses would be filled with idols. Everything would be named after horrible acts and meaningless deities. The places would be impure, and would have a tremendously negative influence on their families. It would be damaging for their children to live in these cities. Therefore, their real major task was to rebuild all the cities for their families. This way they’ll have shuls and schools and study halls. Places where they can properly observe the Torah and its mitzvos. Only then should they build shelters for their flock.
What was the source of their mistake? Chazal are pointing out to us that we see it from their main request. Their focus was concern for their flock. The whole reason they wanted to settle in this particular land was because of their abundant flock. Since that was their focus, they failed to realize the spiritual danger that there was in leaving their families in the conquered cities as they were. This is what it means that they cherished their money more than their families. More than that, the way it is phrased is that that they cherished their money more than נפשות, which literally translates as souls. Meaning, they cherished their money more than their souls, by not being concerned for the spiritual dangers that came from those conquered cities. Moshe taught them that their priorities were incorrect, and they quickly accepted his rebuke.
 Based on Be’er Yosef to Numbers 32:1-38
 Numbers 32:16, 24
 Bamidbar Rabbah 22:9 and Midrash Tanchuma Mattos § 7, brought by Rashi ad. loc. See also Rashi to Proverbs 20:21
 Numbers 21:23, 22:33
 Bamidbar Rabbah 19:29, Midrash Tanchuma Chukas § 23, and Tanchuma Yashan Chukas § 52, brought by Rashi to ibid 21:23
 This is also the intention behind Deuteronomy 12:2,3