He can't say he doesn't enjoy, in some small way, having her so attached to him.
Of course, Joker doesn't like Michelle at all, not really. He thinks she's amusing enough, but ultimately, she's like a wind-up doll: somewhat nice to look at, worth some short-lived laughs, but completely useless otherwise. She's useful for two things, really; she's a walking bomb, and she's an extra pair of hands when he can afford to have a very useless goon staggering around like a moron.
What's worth the trouble of dealing with her, however, is what she could be.
The Joker, he can see talent. Not really talent, per se, at least not in the way others think of the word. Other people might replace 'talent' with 'psychoses'. He knows when he finds somebody with the potential to become something as wonderfully twisted as himself. Sure, you can drive any normal, well-adjusted person insane if you apply the right pressure; look at Harvey Dent. But when you want to drive a person insane and leave them coherent afterwards, you can't just slam crisis after crisis on them; they crack, and their brain turns to goo, and they sit in a corner and babble to themselves all day. You have to do it in a particular, careful way. It's like chess with the human mind.
He sees potential in some people, better than any of those psychiatrists at Arkham could. He might've made a good psychiatrist himself, if he hadn't taken to chaos and mayhem instead. He saw potential in Dent after his girlfriend went up in flames. He saw potential in Michelle, too.
It was a complete twist of fate that he even noticed her in the crowd at the Wayne party. One bad smear of lipstick and her entire life is in ruins; that's why life is so hilarious. You can smear your lipstick and become the newest chew toy of Gotham's premier psychotic. He saw the flash of red, and then he saw her smeared smile, and he thought she was making fun of him, playing him off as a joke to be laughed at. It wasn't that she was imitating him, because there are tons of people in Gotham city that idolize him; Arkham crazies that look up to him as a sort of hero of their kind, which is completely disgusting to him, honestly, teenagers that think he's the coolest psycho in the city, pickpockets and slime that admire the fact that he has Gotham City by the scruff of its neck like some sort of stray gutter cat. It's the idea that someone is laughing at him, not with him.
It pissed him off.
So he was coming up to maybe cut her throat, scare off the other partygoers deciding that they were big boys and girls and that they could take him down with broken champagne glasses and kebab spears. He would have sliced her neck, too, if her boyfriend hadn't shoved her. He found that kind of funny. No loyalty anymore, not even in the high class. Not that there ever was any loyalty to begin with. The upper class is like the Gotham Mafia; no honor, no respect, no rules.
He rolled with it. Put a knife to her neck, left smoothly after Batsy took a swan dive after Dent's squeeze. Thought it was kind of funny that she was still able to quip a witty reply back to him on the elevator ride down, thought she might like to die outside instead of in an elevator. An elevator would be unpleasant to die in; look at your reflection in the metal, blood spurting from your jugular, eyes going blank and glazed. He felt like getting some fresh air anyway.
When he tossed her to the pavement, he was going to let the goon handle her, because he thought that Chuckles or whatever his name was could handle a helpless woman in an evening gown. Apparently, his thugs aren't as capable as he thought they were, because she pulled one of the oldest tricks in the book and outsmarted the guy. That was reason enough to kill him. But when he blew out the man's head, and getting ready to blow hers out too, he saw her have a kitten at seeing the body, and he thought she might be worth a laugh to have around. Maybe he could mess around with this one before he slit her throat and watched her bubble and drown in her own blood. He dragged her back to the base. Decided to torment her for amusement in his down time.
Then he found out that she was terrified of the dark, of being roughhoused, of blood, of so many things. A wonderful cocktail of neuroses. He saw the potential there for some sort of weapon, though it wasn't something he was going to aim for. A pathetic, terrified woman that couldn't help herself, that cried and shivered and played princess that needs rescuing. Disgusting. He didn't want a tool so brittle. She'd make a good human bomb though; that's an honor, in his book. Bombs are one or two levels above goons.
After the visit to Gotham PD, and a glance at her file, he knew that she might be worth something. A woman traumatized, scorned, sex slave for three years that beat her captor's head in when she got the chance. She can fight back when she's pushed; she just needs that little push.
He's a good pusher.
That's really when the Joker decided to try and see what he could create with Michelle's canvas. More like a pet experiment than anything else; if he fails and ruins her, oh well. If he succeeds, then great. He locks her in the pitch black room when she gets mouthy, when he does finally find out that she can stand up to him outside of complete safety. He waits. He finds out that she's good after he does this to her, like a dog with its nose rubbed into a mess it's made, or spanked with a newspaper.
After Batsy gets him captured, locked up for good this time, he asks about her just so that she knows that he hasn't forgotten. Why abandon a project half-finished? If he doesn't finish her, then she's going to live her life in misery, and if he breaks her, then she'll die and he won't leave any loose ends. He's sure that he's doing good on his part to haunt her, even as he sits in Arkham and waits for that sudden idea of how to get out of the asylum; it's going to hit him randomly, in a moment of complete boredom. It does. He escapes. Too easy.
He finds out where she lives. Finds out that she goes to a psychiatrist and complains that her life is so horrible, the world seems so unfair, and the people living in it are dogs. She's seeing it like he does, just like he said she would. During a 'visit' to her home to see what sort of protection against intruders she's undoubtedly set up, he finds her two rather large, slobbering, foaming at the mouth dogs. That's a setback.
Once he decides to take her back and fix her proper, he draws his little message on her work building and banks on her coming back impaired somehow, in one way or another. It pays off. While she's gone, he visits her house, and feeds her mutts some glass-filled hamburger. The stupid, bigger one eats most of it and he knows the pooch's days, minutes really, are numbered. He goes to break in, wait for her, set the mood really, and the one that ate most of the glassy hamburger attacks. It sinks its teeth into his forearm and he beats the shit out of it to pay it back, gouges out its eyes with his thumbs, breaks the dog's back legs, kicks out all its teeth, and watches it blindly drag itself, whimpering, bubbling up blood, under the couch. He laughs at it as it does. Never liked dogs.
The other dog comes next, and he grabs the animal and drags it to the bathroom, digs a switchblade from the collarbone down to its hind legs, and hangs it from the shower head with a daisy chain of socks. It's a hilarious sight. He doodles on the mirror, so that she can look and see him waiting for her and it turns out perfect, and he waits in the corner of the room.
Michelle is just as stupid as he thought. Maybe not stupid, but lacking in common sense at least. She plays into the trap just like he thought she would, though the mercy killing of her dog is something that makes him have to stifle his giggles. So overdramatic.
He takes her down, and is surprised enough at how combative she gets on the ride home. He has no problem knocking her back down to her level, which is about boot-level, and he keeps chipping away at that shield of hers. Chip, chip, chip. Newspapers, his own wonderfully twisted view of the world; he knows that she's dangerously close to her wonderful breakthrough.
It's when a cop lets out a tell-all, very candid statement to the newspapers. Joker knows that this is it. This is his ace in the hole; this is the joker card that's going to smash her into a million pieces.
He's right.
Michelle King breaks. She snaps. She shatters like glass. She tells him that he's all she's got left to her, and he doesn't doubt her in the least. Who's been there for her when everyone else abandoned her? He has. He's nice to her sometimes. It's not often, but it's still sometimes. He's like a friend to her, a real friend that listens to her when she talks, and teaches her about the world; he's a teacher and a friend and he's all she's got left. She's going to need him or she'll go gibbering mad.
Right now, as he walks down the hall with her under his arm, since she's not letting go of him for anything right now, the Joker knows that this woman is going to be his very best tool, at least for a little while. There's a good chance that he's going to eventually have her blood on his hands, and her corpse at his feet, and if, when that ever happens, he's going to laugh. Laugh, because he made her, and he's the one that destroyed her.
He still needs to meld her into his ideal shape, though. It's going to be a process, like everything is, but it's going to pay off in the end, if not for usefulness, then for amusement.
"Michelle," He begins in an easy tone, "What do you want to do with your life?" The question rolls off his tongue easily; this answer is going to be a very powerful deciding factor in what he makes her into. She keeps walking with him, ignoring the odd looks from the goons and thugs as the pair walk past, and looks up to his face with a childish expression, though her face is still ugly with puffy red eyes and blotchy cheeks and makeup running down her cheeks.
"Anything. Just…please, don't leave me. I don't want to be alone."
Joker smiles at that, because he knows that he's got a perfect tool in his hands.