Mattos / Masei

As they near the land on the other side of the Jordan, the tribes of ראובן & גד ask Moshe for that land, instead of their rightful inheritance in the land of Israel.        
Moshe gets angry & says, האחיכם יבאו למלחמה ואתם תשבו פה "Shall your brothers go out to battle while you settle here?" Moshe goes on to admonish them that they're potentially destroying the moral of the people. He says, this is what your fathers did when they sent the spies. He then continues to recount all the calamities that befell the people because of the spies.
Since they had no intention of not fighting together with their brothers, & even offered to lead them in battle, why would they let Moshe continue to rebuke them & not say right away that they were going to lead the fighting?

There's a story told about the Sfas Emes when he was a young man. One night he learned through the night with his friend, davened at dawn, then lay down to rest for a little before his morning seder with his grandfather the Chidushei Harim. He overslept & came a little late. Immediately his grandfather gave him a lengthy lecture about the value of time. When the young Sfas Emes made no attempt to excuse himself, & just took the criticism without comment, his friend asked, why didn't you tell your grandfather why you were late? Tell him we were up all night learning. He replied incredulously, "And miss out on a musser shmuz from my grandfather?"

The Sfas Emes took out a chumash & showed his friend this story, & asked him why didn't the tribes of ראובן & גד stop Moshe's rebuke right away? He answered, because they wanted the rebuke from their rebbi. Sometimes even if you don't deserve it, a little musser can't hurt.  
As it says in אורחות חיים לרא"ש , rejoice when you hear rebuke like you found a great treasure. Rebuke a wise man & he will love you.

Now that might explain why the tribes of ראובן & גד didn't stop the rebuke, but why was Moshe so harsh on them? Why didn't he give them a chance to explain? They never said they weren't going to fight, & he did give them the land so there was obviously some merit to their request.    

Rav Dessler explains that Moshe was upset because their priorities were messed up. They saw the land & were more concerned with their livestock than their families. They were willing to forgo their spiritual well being living with the rest of כלל ישראל just for the sake of their material needs.

It's interesting to note that originally only the tribes of ראובן & גד  came to Moshe requesting the Transjordan. However, when he gave them the land, half the shevet of מנשה was suddenly included. How did they get involved?

Even after moshe got upset & admonished the בני גד and בני ראובן, they continued to have their priorities messed up & said we'll build pens for our flock & then homes for our families. They were still putting their possessions before their families. Moshe saw that they didn't get the message, so he sent half of מנשה, who were great torah scholars, along with them to mentor & teach them so that maybe they would learn to listen to rebuke & straighten out their priorities.   

We have to take a lesson from them, to put our families first, & not let anything come before them. We should be able to listen to others who might know better, and be able to take rebuke & mend our ways. We have to have our priorities straight & not sacrifice our spiritual pursuits for material ones, enabling us to grow in the service of 'ה!