I constantly make these posts early in the morning when Im sleep deprived and struggle even more than usual to put my thoughts into words but god Desire's existence is just so fucking tragic to me
Ren and Aoba as Restraint and Reason could exist as themselves and live their lives as their own people dog for Ren most of the time, but he still becomes his own entity but Desire could never hope to thrive in the same environment. Aoba and Ren can exist passively, grow and evolve and become more than a manifestation of their namesakes. But Desire's very nature is all-consuming and destructive. He's a bottomless pit of want want want take take take regardless of what it costs or how it may hurt him and if left to his own devices he would slowly chip away at himself until there's nothing left (Shiroba prime example)
Desire can't live in harmony alongside Aoba and Ren (as they do with each other) because their very natures contradict his own. Restraint and Reason exist to limit his influence because the relentless pursuit of unfiltered desire is inherently dangerous. He may be pure instinct but survival is on the back burner and if we take him at his word in Mink's bad end he doesn't care about living as much as Aoba does
Desire's only options are either the living hell that is being trapped within his own mind or losing himself to his own drive until he gets him and Aoba both killed. There is no world where he can both live and be happy. The closest he gets is the True End when Aoba finally accepts him as a part of himself, which in and of itself is a form of death (the loss of his individuality) side note; incredibly fitting with the overarching motif of Aoba and death
I think that's part of why Desire wants Aoba to accept him so badly. Life itself is a burden he simply wasn't made for. Desire's birth was like granting life to a campfire, he either burns and burns and burns until there's nothing left to burn, or you douse the flame before it fizzles out.
They both reach the same conclusion but the death of acceptance is much more kind than the death of rejection. Acceptance grants mercy. To be accepted is to be loved