This song is about, like most of their songs, about a relationship between two people. In the first verse, the "narrator" of the story has found a note saying that his (I assume) girlfriend has left him. He can't sleep and he's finally noticing that everything he owns isn't as good as he thought it was, and his girlfriend leaving has finally told him that, as hard as it is to believe. In the chorus he exclaims that she left him without a word and he just can't stop thinking about the two of them. He goes on to talk about how everything seems blank without her and that everything they built together must have been a lie. He wonders why she couldn't just try to make it work between them. He says that the pain she caused him is slowly killing him from the inside out, though it's also what's keeping him alive because he knows that even though it hurts, he still feels something. His heart hurts but he's fighting back through the pain. As the final chorus comes back, he's just repeating that he's still obsessed with the two of them and they're all he can think about. The title "By The Way" is representing the notion that this is what he'd tell her if she was listening and that he's trying to brush his pain off of himself as if it was nothing.