The fact that he's continuing to get attacked makes him angrier.
I've gotten bit by spiders, flies, and mosquitos and not realized it. Yet my actual skin was pierced and blood (in some cases) was drawn.
The pain of their bite didn't make me angry--it was the fact that these things were bothering me ("attacking" me/not leaving me alone) that made me angry.
I'm not saying the Hulk doesn't feel pain. If Thor or Juggernaut hits the Hulk, it hurts! But I think that something that doesn't even break his skin isn't "painful". Annoying, yes...but I'm not sure a human's bullets would cause him pain. Iron Man could probably cook something up to hurt him, but standard machine gun fire, I don't see hurting him. Just my opinion, of course.