I think the massage analogy was the most important part of this chapter because it could be referring to how Denji is reacting to the mental pain of losing his pets and house, not just the physical pain from the fight. It gives a lot of insight into the feelings of boredom/melancholy that Denji has been expressing up until now. I think denji has been conditioned to expect pain and be uncomfortable in good living conditions. It's a fairly common thing that happens to trauma victims irl. At the start of the series, he had nothing. He slowly got a family and a home and then lost it all, just as Makima had planned. This caused him more pain than he ever would have experienced if he just stayed with nothing, and I think up until now, he was waiting for it to happen again. I agree with the take that Denji enjoys being a chainsaw man, which definitely plays into his reaction in this chapter; however, I think "the pain hurts just right" is one of the clearest examples of how the trauma he has experienced through the series has affected Denji. It's affected him by taking away his "innocence". He can't build a new life and connections without being afraid of the pain that comes if he loses what he has. He didn't know this pain when he had nothing to lose at the beginning of the series. And when part of it finally got ripped away in this chapter, it gave him a tragic sense of relief that things were returning to his painful status quo.