What does Ratty have to forgive Mole for at the end of chapter 1

Mole asks Rat if he can try steering the boat. Amused, Rat insists it is harder than it looks, and promises to later give Mole lessons. Mole is upset by Rat's refusal, and tries to prove his strength by pushing Rat out of the way so he can steer the boat himself. Rat was correct about the difficulty, though, and the boat flips over. Mole drags himself to land, embarrassed of how rudely he acted towards his new friend. However, after diving to fetch all his supplies, Rat forgives the younger animal, and invites Mole to live with him as long as he likes.