What does Rosencrantz have against ambition exactly? First, it’s the thing that turns Denmark into a prison from Hamlet and now he thinks it’s as insubstantial as a shadow’s shadow? Which is it? A really powerful transformation of experience or a light airy nothingness? Either way, it’s not a positive thing. I guess Rosencrantz doesn’t have any ambition himself? Or is he trying to hide it? He is after all, the friend of a prince. He’s also signed up to be a spy on that friend for the king. One could see that as a kind of ambition. Maybe Rosencrantz is VERY ambitious and railing against it to divert attention from it.