What the friend sees
Turn the phone round. Your friend sent the cake on Tuesday. She'd spent the morning on it and was pleased with it, and she sent it to three people, and you were one of them because you'd have got the joke about the icing. She saw that you'd read it. She thought, they'll reply tonight. On Wednesday she thought, busy. On Thursday she didn't think about it. On Friday she saw your name in a group, posting about something else, and thought something she didn't quite put into words. By Sunday she'd filed it. Not as a wrong, but as a small fact about where she stands.
I want to be careful here, because the book so far has argued that the friend isn't hurt, and mostly they're not. But there's a difference between hurt and noticed. Andrew Hales and colleagues asked people to recall a time when a friend checked a phone during a serious conversation, and compared them with people recalling a conversation without that. The ones who'd been briefly ignored reported feeling excluded and a small pain, and the effect partly ran through a drop in how much they felt they mattered to the friend. That's a study about a phone in a conversation, and the earlier book Look up is about that habit. I'm extending it to the unanswered message, and the extension is mine. But the mechanism it describes is ordinary. Being ignored is felt, even briefly, even by people who tell themselves it's nothing, and what it touches is the sense of mattering.
An open message is a small, repeated, silent version of that. The friend doesn't feel it as a blow. They feel it as a slight lowering of the temperature, an adjustment to how much they'll send next time, a question they'll ask a little less confidently. Over months the adjustments add up. The friend who used to send you cake photos sends them to two people. The one who asked about a drink three weeks ago stops asking, not from pique, but because they've learned what happens when they ask. They didn't decide any of this either. They adjusted, which is what people do.
What the friend never sees is the meaning-to. They don't see you opening the message on Wednesday and Thursday and Sunday and feeling warmly and putting it down. From their side, the warm feeling and the cold silence are indistinguishable, because only one of them arrives. That's the unfairness at the heart of this habit. The friend is being treated, from their end, like someone you've cooled on, by someone who hasn't cooled at all.