He's not crazy, he's smart enough to make people including Batman think that, this is his biggest Joke of all, and it's on all of us. Think about it.

@Hondo_ said:

Isn't that an oxymoron?

I can see why you'd think that.

Me, I always figured because his mind was so abstract he could see things from different angles,kind of liek looking at a icture and everybody notices the man in the front but not the dog in the back.

Madness and genius are two sides of the same coin, yadda yadda yadda...

@Mercy_ said:

The ignorance in this thread in regards to mental illness is troublesome.

This... lots of this...

He is not insane.

@Glitch_Spawn said:

As I just told CM earlier, the criminals to really fear are the ones who know exactly what they are doing and are okay with it. Someone on a psychotic break isn't very aware of their surroundings, but The Joker has that love for chaos in a way only a sane person can love something. He obsessively loves destroying anything and especially anything that has a place in Batman's heart.

This sounds about right.

I have Aspergers, ADHD, Bipolar, and can't sleep without meds. That makes me retarded and and and like dumb and stuffs. Anyways I'm going to go eat a movie and watch some chicken wings!

Being insane doesn't make him unaware of what he's doing.

Questioned is now answered.

@Ravager4 said:

@Mercy_ said:

The ignorance in this thread in regards to mental illness is troublesome.

It makes my brain hurt.

A bit but it makes me wonder how do you know about mental illness?

As others have mentioned, "crazy" people are often very intelligent. In fact, in many, many cases, extreme intelligence seems to correlate to a breakdown of sanity.

@ReVamp said:

Questioned is now answered.

this

@Mercy_: It's enough to drive one mad.

The Joker persona was made to go against order as he viewed it boring and is easily breakable. When he sees Batman, a person that dresses like a bat to scare criminals to bring order to Gotham a city that was already in ruins, he sawed it as foundation for himself a way to make his identity. You have to wonder his morally opposite the Jokester would work.

Joker is not crazy but desires disorder and my guess is disorder always wins in the end bec. Joker has the universes and the capricious gods on his side.

All geniuses are a little crazy.

@arnoldoaad said:

Crazy?

this, this, and freaking this

You know with the characters like ice king showing a better example of crazy that people expect. Joker walks the line and seems close to crossing it, or did he already. He can be highly impulsive. The debate is when does he snap and when does he recover. When he thought he was going to die he made jail fill with super villains into jokers. The joker is an alter ego that takes mental breakdown an uses it as inspiration. It is hard to tell when that happens. When is he crazy and when is he not?

I don't really find it useful classifying human behavior as crazy and not crazy since all human behavior is quite stupid anyway. Both Joker and Batman are just plain weirdos like most humans.

Simple answer: somebody wrote it that way 
I'm not here to debate mental health issues, but I will say this much. There are a lot of people commenting that being "crazy" doesn't make someone dumb or intelligent, which is to say that mental disorders don't affect intelligence. Yet there are quite a few saying that many genius or highly intelligent people are "crazy". Everyone can have their own opinion. The question was why is the joker smart if he is crazy? In my opinion a better question or questions is:  was he a man of normal intelligence before he became the joker, and if the answer is yes how does becoming the joker make him a damn genius?