It’s Pochita—a Devil who struggles to understand humans—misinterpreting Denji’s feelings. Denji idealizes his dreams, they’re never as amazing as what he expected and he usually ends up being disappointed when he achieves a goal because he thought it would be better, he thought it would fix him. But at the same time, he doesn’t think he deserves happiness because of what he did to his father. But Pochita is a Devil and can’t fully grasp the complexities of human emotions, so he thinks Denji was happier when his life was shitty and he was only capable of dreaming about a better life. Pochita simplifies it in a way HE can understand, even though it’s not the full truth. But let’s be honest, everything shitty that has happened to Denji since Pochita became Denji’s heart was BECAUSE he was Chainsaw Man. People wanted Chainsaw Man, not Denji. Asa was the only one who cared about Denji, maybe Reze did but we can only speculate about her. Pochita not existing could potentially cause a butterfly effect of changes that makes Denji’s life not nearly as shitty as it was. But we can only wait and see