Inexperienced negotiators know that haggling feels bad, so they try to “play it straight” and offer their “best” price first, graciously allowing the other side to get their lowest offer without any unnecessary grief.
Experienced negotiators know that it feels bad to simply say “fine” in a negotiation, without getting at least a little concession from the other side, so expect the offer to be made with some reserves built in.
You can see why the saying “the road to hell is paved with good intentions” starts to apply here. Both sides are trying to avoid conflict, and expecting the other to do the same. To assume otherwise would risk underestimating the other person, which is rarely a good idea.
Now, this is why this doesn’t work as intended:
All negotiation is an extension of a relationship between people, so they are also very sensitive to assumptions, biases, and emotional “damage”. Since what happens in another person’s head is unfamiliar territory, we tend to assume they are looking after their own interest, in exactly the same way we do. When these assumptions happen to clash, problems occur.
That’s why it’s always safer to assume that the other side expects you to have reserves in your initial offer: so that if some concessions are made you will still be happy with the deal. That way, if you are proven right you are safe, and if you are proven wrong, even better.