User blog comment:Monkey d strawhat/True colors/@comment-3119203-20120210015014

I don't think Shanks wants Teach's head. A clash between the Yonko is tantamount to the end of the world, so not yet at least.

I seem to specifically remember Shanks saying that Luffy would return the hat when Luffy was a better pirate than Shanks, so of course he has to prove himself.