In answer to Moshe's charges, the two tribes answered him by saying that they would build pens for their sheep and cattle, and cities for their wives and children on the eastern bank, and then they would go with their brothers to conquer Eretz Yisroel, and even volunteer to be on the front lines. They also said that they would stay in Eretz Yisroel until all the other tribes had received their inherited portions of the Land. (It took seven years to conquer the land, and another seven years to apportion it to the tribes). Only then, they said, would they return to their families on the eastern bank of the Jordan.
Rashi comments on their response, noting that they first said that they would build pens for their livestock and then cities for their families. He understands this as meaning that they were more concerned about their livestock than their own children. They should have first said that they would build cities for their families, and then pens for the animals. Moshe, in fact, corrected them - when he agreed to their proposal and reiterated their conditions to them he changed the priorities of what got provided for first.
My Rosh HaYeshiva, Rav Henoch Leibowitz, of blessed memory, asked how such righteous people as these tribes could care more about their livestock than their own children. He explained that originally they were naturally more concerned about their children, and that's why they worked so vigilantly to raise and increase their livestock, motivated by the desire to provide for their families. As time went on, though, they became more and more absorbed in their livelihood, and began to look upon their livestock as a value and purpose in and of itself, not just as a means to provide for their children. This eventually led them to become more concerned about the welfare of their livestock than their own children.
The Midrash Tanchuma takes it a step further. It says that the tribes of Reuven and Gad were not just more concerned about their livestock but that they actually cherished and loved their livestock more than their children. How can we understand this? It is human nature that we generally love that to which we give more of ourselves, that to which we invest more of ourselves. The two tribes' preoccupation and deep involvement with their livestock and livelihood eventually led them to cherish the animals more than their own families.
Have a Great Shabbos!