Frustration. Imagine if Wlad would have grown angry enough to hit him with his fist during the fight?
Disappointed to see that. I didn't find Klitschko's way of boxing very sportsmanlike already before that incident, but still I wouldn't have expected him to do an intentional headbutt. What makes it worse is the image Klitschko has built up in the past, the image of a clean and fair sportsman. With that image in mind, such an action is even worse, because it not only makes him a dirty fighter, but it also makes him look like a fraud.