"Yeah, if I wind up going back from Riverview. What's to stop something from happening like with you hopping around from world to world?" Of course he's going to ask how he's so sure about that. Does he not know himself? "Who's to say if time is still going to be frozen for me in Amestris when I'm stuck here?!" Making false promises was a quick way to piss him off. Then again, there were a lot of quick ways to piss him off.