User blog comment:Omega natsu2/Welcome Back, Frosch/@comment-121.54.54.138-20140307094813/@comment-7418318-20140307102042

Even when he was seemingly emotionless, he still got emotional when Frosch returned to him after the fight against Gajeel.

And during the fight with Natsu he did seem to get angry, when Natsu challenged them to a 2v1.

So him getting annoyed at Sting in the beginning and getting highly emotional when the matter is connected with Frosch seems appropriate.

Besides, this just a funny little (not really little, but you get my drift) Omake and not really part of the main story, so don't take it so seriously.