People commonly stop giving reasons for something if the other person agrees.
Say you ask someone to drive you somewhere tomorrow, and you tell them a reason you want to go, and they agree. Then you won't't tell them all the other reasons you want to go. There's no point, since you're already going.
This can be a problem if they change their mind. They may cancel on you and think, "well, it's not a big deal, he's only missing out on [the one reason you gave for going]".
That's bad and irresponsible. If they'd told you they were considering cancelling, you would have told them your other reasons. They shouldn't have assumed that you'd given them a full list of all the harm they'd do by cancelling. they didn't ask for that list (and probably didn't want to hear it, would have found it boring and pointless while they were still intending to drive you).
For example, I might ask to go grocery shopping with my friend today. We might plan to go together. And he asks me why, and I say I need milk.
Then later he comes home late. And I'm like "it's a bit late, but let's go shopping now!" And he's like, "oh don't worry. i already went shopping. i got the milk".
and you're like "wtf? i wanted pastrami too. you didn't tell me you were going. you changed plans on me without knowing all my reasons for wanting to go shopping."
people routinely do stuff like this without even texting you to check in, or ask if you want anything else while they are at the grocery store.
they are trying to solve what they think your problem is, without really giving much thought to what you actually want. it'd be better if they asked you more about it and recognized your life is pretty complex and you probably want several things and they don't know all the details.
Sometimes misunderstandings about giving only some of your reasons come up with big issues with long term consequences. Sometimes you ask someone to do something that's super important, and they agree, and then they don't do it and a large amount of harm is done. And they didn't realize how important it was.
You might think you could fix this by saying to them, "please understand that this is extremely important to me, and i haven't told you my full reasoning. if you have doubts, let me know and i'll explain more." But that often doesn't work.
People usually won't really take seriously how important it is without knowing the reasons it's important themselves. They won't have something really compelling in their mind. They'll remember you said it was important, but they'll feel like it isn't important to them since they don't see the importance. They may start thinking you're mistaken. But they'll do this in a subtle way where they maybe don't realize anything is happening. Often they change their mind and don't realize it until after they're pretty strongly against what they'd agreed to!
People have a hard time emotionally recognizing and feeling something is important just from the abstract concept that there are other reasons they don't yet know. That's a common mistake that gets people hurt. Hopefully people who read this will do a better job with it.
And people who read this can also have more patience to listen to and appreciate reasons for things you already agree with. Try to remember you may change your mind later and do a bad job finding out more about it at just the right moment. If you aren't super good at asking for more information really early in the process of starting to reconsider – a very rare skill – then it makes sense to find out more early on while you're talking about it.
Especially if it's important, try to find out really thoroughly about why, and care instead of being bored. doing otherwise is risky.
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Acknowledgments: some ideas presented are modified from, or inspired by, ideas from Karl Popper, Ayn Rand, David Deutsch and William Godwin. Thank you!