First of all, let's get this straight. I don't truly believe he has bipolar disorder but that he may have been diagnosed with it upon entry into Arkham Asylum.
Bipolar disorder is defined as 'a mental condition marked by alternating periods of elation and depression'. That might not mean much to the average chum but looking into what comes with bipolar lies the true evidence to my theory...
From what I understand mania is the centre stage for bipolar disorder coming in different stages of severity depending on the person with it. Mania, when treated can end up only lasting in short bursts but as it takes a form of euphoria, if the person with it is alone for a long period of time the short bursts can increase into becoming part of the persons personality.
The most severe level of mania includes lack of sleep, poor judgement drug abuse, hyper sexuality and excessive spending to name a few. Remember that custom Lamborghini Mr J cruises around in? Could that be a result of bipolar? It's a long shot.
Not convinced yet? Don't worry.
Hypo-manic and manic episodes are the main occurrences in bipolar. In the most severe cases of manic episodes the person can break away from reality developing a state where there thinking is affected along with their mood. They may feel unstoppable or as if they can't control what happens. Another trait can be extremely distorted belief's on the world and society. Joker has been known in the comics to be a likely candidate for bipolar so it wouldn't be far fetched that he would be too in the cinematic universe.
Now remember that bit with the Joker in the Suicide Squad trailer ? Course you do.
Those things he's holding are used for electroshock therapy which can be used to treat severe cases of bipolar when all else fails or the person refuses other treatment. Let's say Harleen Quinzel is an ex-girlfriend of Joker before he's admitted to Arkham. Or let's not. Either way we can presume either her or someone else diagnosed him with bipolar disorder and started treatment right away. One form of treatment is to simply talk to the patient to form a 'therapeutic alliance' in support of recovery. If Joker and Harleen haven't met before,this could be how they bond. If they have met however I'd assume either one or both of them would be too pissed off to talk.
So continued treatment isn't helping and Joker isn't presenting signs of wanting to change his ways. Electroshock Therapy it is. Again going back to if Joker has met Harleen she may authorize EST out of hatred. If not, she may authorize it due to wanting him to show progress in hope of a life with him outside of a cell after falling for him.
Cut to the breakout.
So the Joker's been busted out of prison and is on his way back to being the clown prince of crime. But before he leaves he decides to tie Harleen to the EST chair. Is it to push Harleen over the edge and finally become our beloved Harley Quinn? Or is it punishment for breaking up with him and authorizing the EST?
Either way,is this just a glimpse at the start of the abuse towards Harley or is it a twisted take on the origin of the Maiden of Mischief?