Because they've grown up in an entirely militarlised society, a society where they are basically bred to be in the military, and their whole media and everything says to them that the world "hates" them either for no reason, or because they're jews. Notions which in the west you might think have no meaing any more, but many of these people have originally come from places like moldova, which had jewish communities which were completley devastated by the holocaust and never recovered. The majority of israelis are also not the sickenly rich zionist supporters and american "jewish" settlers we see on tv programmes in the west, they are actually themselves rather impoversished which is not to compare it to anything like the palestinians' suffering, but it might explain a bit why israelis feel so defensive about whats going on there, and the nationalism...
