why were Max's adolescent peers surpised by his aggressivness?

"Basically because Max fights so much. As a teenager Max continues fighting among a group of friends and enemies. Max fights a kid named Walter Kugler and wins; the two go on to fight thirteen more times, and they become good friends."

The stereotype is that Jews are not aggressive.

http://www.gradesaver.com/the-book-thief/q-and-a/why-were-max-s-adolescent-peers-surprised-by-his-aggressiveness-113585/