why do people hack?

This question is too vague. It's like asking "why are you wearing those clothes?"

People hack for all kinds of reasons, and there are many kinds of hacking. Some people do it for financial gain, for blackmailing, to obtain secrets/private info. Others do it to improve security, help businesses, find criminals. There are good reasons and there are bad reasons.

So "why do people hack?" - I don't know. Why do [insert some action which people sometimes do here]?