Warning: Spoilers for Portal 2.
Wheatley had been told many things when he'd first been brought online.
For one, that if he ever left his management rail, he would die. The same thing went for his flashlight. He was told his death if he did this would be slow and painful - well, pain as he was programmed to feel pain, anyway, which was surprisingly much like he had seen countless humans suffer pain in the testing chambers. Screaming, shaking, crippling pain. He'd seen firsthand how cruel GLaDOS could be to her test subjects and so hadn't ever questioned when he was told that if he did a certain thing, he would die like so many of the little humans had.
Another was that above all else, he was to obey all orders but feel absolutely free to chip in with his own ideas. If he felt that an idea of GLaDOS's was flawed or needlessly cruel, he had an attachment directly to her brain that would allow his own ideas to be heard. He took advantage of this more than he would have liked to admit. It made him feel special to know that he was being listened to, especially by someone as powerful and frightening as GLaDOS herself.
Third - and this was important only because he was in charge of the little bleeders - was that he was not to grow too attached to the test subjects.
"Test subjects," all of whom slept - slept, yes, not offlined but slept - soundlessly in their overstuffed beds for months, for years, until GLaDOS determined that it was time for them to test. They would be reanimated when the cycle called for it to check them for brain damage, made to look at the ceiling and floor, stare at a painting to be mentally reinvigorated, and then calmly go back to sleep until they were needed.
And Wheatley was put in charge of the lot under the condition that he wasn't to grow attached.
And of course he'd listened. Yes, he'd listened - at first.
Chell had been nobody to him when they'd first met. Funny, considering that he'd watched her do her gymnastics exercises for nine nine nine nine nine nine nine- For a long time. He'd watched her for a long while by that point, but she had been simply one among ten thousand other humans. She'd been brain damaged too, looking like hell dragged up out of the metaphorical rubble, her hair frayed and jumpsuit tattered and her slender body weak and pale from being in stasis for so long. She'd been nobody. Mute, dark, with bruises around her eyes... but nobody.
Not until suddenly she listened to him. Not until suddenly she asked for his opinion in his silent way. Not until he was risking his own life and limb to help her, bringing GLaDOS back online in his fumbling desperation to get them out of there. Oh, and Chell wasn't a nobody after all, was she? No, she was the one to bring down GLaDOS single-handedly. She was the one who had brought Aperture Science to its figurative knees.
Still, this human depended on him even when making her knobby-kneed way through GLaDOS's tests. Wheatley struggled and fought to get even a glimpse of this girl, this foolish human with the weight of the world on her thin shoulders. Just a tiny peek of sleek dark ponytail against narrow shoulders. The sound of that portal gun firing for safety.
She depended on him. That had frankly never happened to him before, and he found that, foolishly, he needed her too.
He detached from his rail for her. Turned on his flashlight for her. Grew too attached for her. Did all of the things that his creators had programmed him not to do, and felt a strange sort of rebellious thrill to do it. They were going to bring GLaDOS down together or not at all, he realized, and knew that there would be no one else in the world he would rather do this with.
So imagine his surprise when he found out it was all a lie.
Thanks to his previous job of watching over the subjects - and that's all they were, he realized all too belatedly, nothing special, just lab rats, all of them - he knew the facility inside and out. Knew every catwalk, every crevice, every nook and cranny, every single floor of every single sub-level, every elevator shaft and office block. Every test chamber, every staircase, every trial room filled with defective turrets. Every factory machine. Every robot gone terribly awry.
So Chell couldn't run away.
"I've done nothing but sacrifice to get us here and what have you sacrificed? Nothing. Zero. All you've done is boss me around. Well, now who's the boss? Who's the boss? ...it's me."
It was the first time he had ever seen Chell cry, that moment in the vast grey room he was going to use to destroy her. His once dear friend with her eyes wide and shining and pink around the edges from moisture and saline... it would have been rather humiliating to look at if he weren't so moved himself. Part of him said that she deserved it, every minute of the emotional agony he himself had been feeling since the moment the humans had decided that the facility's Artificial Intelligence had become too intelligent for their own good. But Wheatley wasn't a moron. No, he wasn't, no matter what GLaDOS or his creators had said. And he hoped, or at least a part of him hoped, that Chell was feeling as ridiculed and ashamed as Wheatley had felt for his entire artificial life.
"Suddenly not feeling so powerful, are ya?" he asked her. Funny, she looked so tiny suddenly from where he was plugged into this massive machine. The raw, aching hunger for tests almost literally burned in his circuits. Withdrawal was hitting, and it was driving him mad. "I sacrificed everything for you... you stupid, selfish little human."
His words were biting and cold, nothing like how GLaDOS had sounded when berating him for his mistakes. No, he could never master her bored, dark humor in things. All he could manage right now was the bitter, cold rage that kept him going, kept him testing, watching this girl lift cubes and fire portals and avoid the deadly blast of turrets. It was all he could do right now not to squash her himself, here and now.
"And for what? For you to- to boss me around! You monstrous, bossy-!" His words failed him again, because saline trickling down her cheeks and he realized that really, he really had never seen her cry, despite what GLaDOS and himself had forced her to do. Placed her under such intense pressures, but she'd never cracked until now. For what? Certainly not for him.
He narrowed his large optic at her and spat, "You never cared half for me what I cared for you," and saw her cringe a bit in... what? Guilt? Wheatley still struggled to gauge human emotions despite himself. "And another thing! You never caught me! I told you I would die falling off that rail! You didn't catch me! You didn't even try!"
Oh god, he cared for her so much. Even now, because seeing her look at him like this, seeing her lips barely move to form words without sound - "I tried" - his entire conscience was screaming for him to let go. Drop everything he was doing here, drop the renovations, drop it all to go back with her and feel her strong arms around his small core of a body once again.
"I wish I could take it all back. I honestly do. I honestly do wish I could take it all back. And not just 'cause I'm stranded in space. Anyway, you know, if I was ever to see her again, d'you know what I'd say? I'd look at her and say: I'm sorry. Sincerely. I am sorry I was bossy and monstrous and... I am genuinely sorry. The end."
Wheatley had been told many things when he had first been brought online. He'd been taught hate, and loyalty, and fear. Rejection, disappointment, shame. Humiliation. Terror. Rage.
But not once had he ever been told that what he'd been feeling for Chell was not friendship, but love.