Howl takes his anger and frustration out on the castle by filling it with ooze, in much the same way Sophie took her anger at the Witch out on the castle by cleaning it. So, though their methods differ, they’re both doing more or less the same thing here. Sophie, however, steps into a more mature and maternal role when she gives Howl milk and tries to talk to him about what’s going on. She can do this because despite being younger than Howl, she looks and acts much older, thanks to the Witch’s curse.