Because Rat is observant and values being prepared, he immediately realizes what happened and how to safely remedy it. He also shows that he’s not going to hold a grudge or punish Mole for disobeying him. Rather, he’s going to gently and kindly teach Mole that setting out on his own was a bad idea, so that Mole knows exactly why he shouldn’t do this again. This makes Rat a good mentor (though, of course, it helps that Mole
wants to please and learn from Rat—unlike Toad).