Peeves just didn't care.
He couldn't tell you why.
He just didn't know how to.
Peeves had spent most of his existence doing what he was destined to do as a poltergeist, just generally cause trouble, he never really thought that he could do anything besides that, he didn't know how to.
That's why he didn't feel any remorse or guilt as he watched countless first years cry themselves through the first few weeks of term because of a particularly nasty prank he had pulled; he didn't care as he watched Filch slave away at a mess Peeves had caused for the hell of it, hell he didn't even care as he saw Remus Lupin (a particularly nice student that had never cursed at peeves) slip and fall all the way down the fourth floor stairwell.
He just didn't know any other way to live.
He wasn't as lucky as the other Ghosts who boasted about their accomplishments during their time among the living, because Peeves never experienced that, having been born of magic.
He even found himself envying Moaning Myrtle who at least experienced the thrill of truly being alive.
That's probably why he went out of his way every year to ruin the death day parties the other ghosts celebrated, because it wasn't fair.
Peeves didn't have a death day, or even a birthday because he just was.
That's also why he easily grew frustrated with 'authority figures' he couldn't believe that these people who were blessed with life would throw it away so easily, wasting their lives away over things that, after all is said and done, wouldn't even matter.
Who cared about who made head boy? Did it really matter?
Exam after exam, but on the day you lay to die, would you even remember the scores you once achieved?
No.
But you would remember other things, more important things.
The aurors who lost their lives protecting others, that was something to be proud of.
The Weasley twins, who despite effectively getting expelled, remained a symbol of rebellion even years after they left the walls of Hogwarts, that was something to be proud of.
The potter boy who saved the philosopher stone from reaching the wrong hands, who ventured into the chamber of secrets to save a girl he barely knew, who saved the soul of an innocent man, who preached the truth to closed ears, that was something to be proud of.
Peeves had nothing to be proud of.
He didn't stand up for the students who were tortured by the Carrows.
He didn't disturb Snape as he sat in his office, undoubtedly brewing evil plans.
He didn't aid the DA in their midnight trysts.
Something he knew he could have done easily.
Because Peeves just didn't care.
He didn't know how to live with guilt or remorse.
He didn't know how to live with pride and achievement.
Hogwarts had always been a family in most eyes, albeit a slightly dysfunctional one at that, but peeves never truly felt like he belonged, an outsider even in the place he had lived all his life.
But, as he glided over the bodies of the dead (faces he recognised as the ones he had once tormented) during the battle of Hogwarts, peeves was surprised to see a small glassy tear separate from his face and slowly glide back towards the ground.
It didn't make sense.
Because Peeves just didn't care.
He didn't know how to.
He didn't know how to live.