Why is Stanley surprised that Theodore insists on being called Armpit?

Stanley is surprised that Theodore insists on being called Armpit because it isn't a very complimentary nickname.
When Stanley is first introduced to his fellow peers, he notices that they all have nicknames. However, all of the nicknames appear to be derogatory in nature. For example, Jose, Theodore, and Ricky are known as Magnet, Armpit, and Zigzag respectively. Even Mr. Pendanski, the D tent counselor, has a nickname. The residents call him "Mom." This intrigues Stanley, but he is not quite sure what to think about the whole practice of giving someone a rude nickname.
After Mr. Pendanski leaves the tent, Stanley asks Armpit where he can fill up his canteen. In response, Armpit throws Stanley to the ground. Then, he tells Stanley that he can fill up his canteen at the water spigot in the shower stall. Terrified by Armpit's violent reaction and unsure of whether there is actually a water spigot, Stanley merely stammers out his thanks. As chapter 5 ends, Stanley speculates that the nicknames may actually be terms of respect.