Wheatley learned very quickly that human pain was far stronger than the simulated stuff he found so frightening as a core. Oh sure, his mishaps as a core were painful enough, whether it was going too fast on his management rail and accidentally crashing into the other cores (that had actually happened more than once), or getting attacked by birds, or being scared so badly he short circuited- not to mention the more severe (shall we say) predicaments he encountered once Chell came into his life, things like core transfers and being crushed half to death- but as a core he possessed sophisticated repair systems, nanobots, replaceable parts, the whole shabang, and so even if something did cause him pain, it was still pretty unlikely that it would actually kill him (not that he'd really thought about that at the time).
Being human was different.
Humans didn't have diagnostics, or nanobots, or replaceable parts, they had skin and bones that were far too easy to break, far too easy to damage, they had immune systems that only worked correctly about fifty percent of the time (if you were lucky), and some sort of self repair system that worked very very slowly, and even then it didn't always heal whatever was wrong completely.
Humans experienced pain on a deeper level for a longer amount of time. As a core pain was more like a metaphorical slap on the wrist, a pre programmed way for a long dead engineer to snap don't do that, to briefly remind Wheatley that he was a very expensive piece of technology, that he didn't want to get broken and cause his creators any more trouble than he already had, now did he? And that was all. Back to business. As a human being pain was much more melodramatic. It was the body's way of panicking, scolding relentlessly like a mother who had just witnessed her child do something stupid and dangerous when they knew better. The words stung because they were true. Are you crazy?! You could have gotten yourself killed! Look what you did! The little voice was accompanied by blood and nasty bruises and cuts that stayed for days, weeks, and far far longer if they scarred. These were visual reminders that That was stupid, You need to be more careful, You know better than that. Human pain hurt more because it cost more, and as clumsy as Wheatley was he felt more than his fair share of it.
And yet the pain wasn't completely terrible, because, for some reason, it caused Chell to be nice to him.
Wheatley wasn't sure where exactly the two of them stood at the moment. She didn't hate him, at this point he was fairly certain of that, but she was still very cautious around him- uneasy- and though she was never really mean to him she had also made it perfectly clear that he was far from her favorite person. Chell never spoke to him (though he was fairly certain she could speak), she avoided his eyes when she could, and she tried to remain stoic whenever they were in the same room.
But when he was hurt it was almost as if she was a different person.
Wheatley had seen Chell on the warpath back There, had seen her when she was destructive and angry and murderous and nothing else. He knew how dangerous she could be. So it was a bit of a surprise, the first time he got hurt, to find that she could also be very gentle when she felt like it.
Chell would kneel beside him, assessing the damage as he squirmed in discomfort at the pain and (for some reason) her proximity. Her eyes would meet his then, and they were full of sympathy, patience, understanding, concern. She was sorry, they read very clearly, and he was beside himself because that was his line. Her touch was light, the washrag she used to clean his injuries ghosting over whatever was hurt so gently he hardly felt it. Her other hand would either entwine with his or offer some small gesture of comfort as she worked, smoothing across his back or briefly tousling his hair. If ever she caused him additional pain her reaction was immediate, she would offer an apologetic sort of grimace as she backed away, allowing him a moment to recover before she went back to work. This was the only time her emotions- her expressions- were truly open.
After a few minutes the pain of his injuries would be gone, replaced with a new one that was almost pleasant as he watched her walk away.