According to AAASHOSE, Joker cannot be classified as insane. Rather, he has a mysterious neurological disorder that enables him to cope with the world around him, that prevents him from becoming insane. He has no consistent personality and can any day become anything he wants and do anything he wants without any constraints. The pressures of maintaining a healthy frame of sanity don't apply to him because he can shape and adapt to something, without the issue of mentally breaking down.
His sense of humour in the face of the worst of things imaginable to a sane person, is a twisted form of positive psychology. So by right, his mind is actually healthy and incapable of falling victim to insanity. Living a normal and happy life is a hard and probably non existent thing. So, why do we do it? Because it's the right thing and logical?
Joker doesn't see the point in it, because his mind is incapable of it. He steers away from the pressures of normality that can, at any given time, break the mind in any circumstance. Not because he wants to, it's just how his mind operates. We wouldn't dare free ourselves in the way he has...because it's "evil and insane"... We can't live life like it's a joke, because we need to maintain our normality and morality in order to effectively operate in a normal life under a normal society - it's insane. We can't go back to how violent and impulsive we were years ago without remembering the rules of law and order, because we'd be deemed evil and or insane by society...Joker operates mentally away from all that reasoning.
So, to us normal people, he is evil and insane. Medically and philosophically, however, Joker is the sanest person on the planet.
Morals don't apply to him, because he sees no logic in them. They're mental restraints imposed by law and order, that hold back the true nature of human civilization, according to his mind frame.
Joker is what he thinks human beings are deep down, and he doesn't choose this. He is what he is by his own mind. Devoid of morals, law and order, logic, imposed by humanity and decency.
Like said, something primal of the early stages of human beings, mixed in with the absurdity of world of the 21st century. Absurdity of existence, absurdity of law and order, good and evil, the 21st century, sanity and insanity, life and death. Joker copes with these social issues by laughing at them, keeping himself sane in the process.
Millions of years ago, we wouldn't of thought he was evil or insane like we do today. We would've accepted his once humanistic nature and true idea of the world...