Crazy and insane are more or less interchangeable because they can both be used to express being enthusiastic, upset or describing something that's demented. The meaning that you want to get across to whom you're speaking with ultimately depends on your tone and the context of your sentence. For example: "You're insane/You're crazy" are both sentences that can be said mockingly, like when you're teasing someone, but they can also be used to express genuine disbelief or used as an insult. That being said, when using these words, whether or not you want them to come across as negative or positive is completely up to you. It depends on your tone and the context of the sentence.

A good question.