Because Nathanos is more than a bit of a dickhead and even if the Alliance wasn't supposed to be opposed to him solely cause he was a member of the Horde, his whole holier than thou "I'm cool cause I serve the Banshee Queen" shtick does get on your nerves even without the hypothetical Danuser self insert aspects.
It's in reference to how his new model instantly beefed him up from the lanky kid he was in MoP as well as a slight goad at Golden.