Some volumes ago, Shanks and Whitebeard where going to fight or something right? What happened there? Also, I wonder if the reason that Shanks and Whitebeard might be at odds is because Whitebeard is trying to make Ace Pirate King, while Shaks is protecting Luffy's interest.

Well, also Shanks was part of Roger's crew, so the animosity might come from there. I wonder what Shanks thinks of Ace and him being part of Whitebeard's crew.