Shrek has just stormed away from Princess Fiona because he overheard a conversation between her and Donkey and misinterpreted what was being said. He thought that Princess Fiona had called him ugly and impossible to love, when she was actually talking about the way she herself looked, at night, under the influence of the witch's curse. He does not look for clarification, but is instead hurt and angry. He rejects Donkey too, and goes off on his own, sad dejected and very angry. Donkey is not going to allow this.