Matt H. answered 09/21/16
PATIENT :-) Elem/Middle MATH and WRITING; HS SAT and COLLEGE ESSAYS!
The super simplified answer is that Athens had at one point helped the mytelines and several other groups defend themselves against a common enemy, the medes. But after a while, the myteline saw that athens was now being less hostile against the mede and in fact were starting to get hostile with some of the groups they'd been helping. The myteline figured,'we're next, so we better rebel, for it is natural and proper that we should revolt against you if you oppress us, the same way we honored you when you honored us.' This was essentially the code of honor understood throughout greece at the time. If you read book 3, sections 8-12 or 13, you'll get a good idea.