You've got Tom who is just handsomely overjoyed, Chris Hemsworth who is making a sort of derp face, and Jeremy who is allowing as much positive emotion to show as he can. And Robert looks like he wants to eat all the confetti.
I think he thinks it's funny how NOT original Volstagg is (and that it's darkly funny that things haven't really changed). Because his reaction to Sif vs Volstagg is actually very different. -- I feel bad for Loki.