What your yes is worth
So far I've counted what the unwilling yes costs you. This chapter is about what it costs the other person, which is more than you'd think. Most of it is my own reasoning, and I'll say where it isn't.
Start with what your yes tells them. If you agree to everything, then your agreeing tells nobody anything. It's like a reference from an employer who gives everyone a glowing one. Once people know that, the reference is worth nothing. When a friend asks, "Do you fancy coming to the lakes with us in October?" and you say you'd love to, she'd like to be able to believe you. If she's known you a while, she can't. She's seen you say you'd love to about things she knows you dread. So she has to guess, or probe. "Are you sure? You're not just saying that?" You'll have heard that question. It's what people ask when someone's yes has stopped carrying information.
A yes only means something from a person who also says no. The friends whose enthusiasm you trust are the ones who've told you, at other times, "No, that's not for me."
Next, consider what happens to unwilling yeses between the asking and the day. They go off. You agreed on Thursday with a smile. By the following Wednesday you're looking for a way out. Perhaps you find one, and on Friday night your neighbour gets a text saying something's come up. Now she has twelve hours to find someone for the tombola, where a no at the bins would have given her nine days. Or you don't cancel, and you turn up, but you turn up ten minutes late and leave at the first chance, and spend the day looking at your watch. The previous chapter gave the finding that matters here. In Weinstein and Ryan's studies, people helped by a pressed helper got less from it. Your neighbour asked for someone to run the tombola, and she hoped for someone cheerful. A prompt no would have let her go and find that person.
I think most askers, if they could see inside your head, would choose the prompt no over the grudging yes every time. They can't see inside your head, which brings me to the third cost.
In chapter 1 I said the cost of an unwilling yes arrives when the asker isn't there. She saw "happy to". She didn't see Sunday. So as far as she knows, you were happy to. You told her so. She isn't being careless with your time. She's acting sensibly on false information, and you supplied it. If she asks again next year, and the year after, and mentions you to the man who organises the Christmas lights as someone who's always glad to help, none of that is unfair of her. Everything she knows about you says it's welcome.
People can only treat you fairly if they know roughly what things cost you. The habit hides the price from them and then, often, resents them for not paying attention to it. That resentment is worth a moment. You may know the feeling of doing a favour with a smile and then being short with the person for a week, or telling a third party how put-upon you are. I'm not scolding you for it. Any real cost with no honest outlet comes out this way. But from the other person's side it's baffling. They asked a question, got a warm yes and were then punished for believing it.
I said this chapter was about the other person, and what it comes to is that the unwilling yes isn't the kind option it looks like. It feels kind for four seconds. After that it hands the other person unreliable help, false information about you, and sometimes a grudge they can't account for. A clear, friendly no is often the more considerate answer, and it's the only thing that makes your yes worth having.