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some do it for fun, some do it for work, and some do it competitively..
a lot of hackers these days go for the reaction.. they love a good reaction from site owners that they have just hacked, especially those that leave a calling card.. but saying that, there are plenty of 'good' hackers outthere that if you speak to them after they have hacked your site (as long as you're not aggressive towards them or arrogant) will actually tell you how they hacked the site, and some will even offer solutions on how to prevent it happening again..
there are good hackers that are helpful and then there's the crackers who do it for maximum damage..