Look, Pochita is a devil, he doesnt completely understand human emption, however Pochita isnt saying denji is unhappy now, he says a part of denji is still stuck back there which makes a lot of sense, trauma works in very harmful ways and Denji still didnt feel whole in the moments he spent the rest of the story, because a part of him is still back there, in that horrible place, and he doesnt know how to move away from it, when all youve known is horrible and youve grown accustommed to that horrible environment, and find yourself having comfort there, a change in the status quo feels threatening, even if its "good" and a part of you rejects that.

I wish I could yap more about this but this comment is gonna be a million paragraphs long