This is because people are bad at making decisions, and have not gotten rid of the harmful concept of "should". The original comment on this topic was claiming that "should" is a bad concept; instead of thinking "I should x" or "I shouldn't do x", on top of considering "I want to/don't want to x", just look at want/do not want. "I should x" doesn't help you resolve "do I want to x", and the second question is the only one that counts.

I think that your idea about morality is simply expressing a part of a framework of many moral systems. That is not a complete view of what morality means to people; it's simply a part of many instantiations of morality. I agree that such thinking is the cause of many moral conflicts of the nature "I should x but I want to y", stemming from the idea (perhaps subconscious) that they would tell someone else to x, instead of y, and people prefer not to defect in those situations. Selfishness is seen as a vice, perhaps for evolutionary reasons (see all the data on viable cooperation in the prisoner's dilemma, etc.) and so people feel the pressure to not cheat the system, even though they want to. This is not behavior that a rational agent should generally want! If you are able to get rid of your concept of "should", you will be free from that type of trap unless it is in your best interests to remain there.

Our moral intuitions do not exist for good reasons. "Fairness" and it's ilk are all primarily political tools; moral outrage is a particularly potent tool when directed at your opponent. Just because we have an intuition does not make that intuition meaningful. Go for a week while forcing yourself to taboo "morality", "should", and everything like that. When you make a decision, make a concerted effort to ignore the part of your brain saying "you should c because it's right", and only listen to your preferences (note: you can have preferences that favor other people!). You should find that your decisions become easier and that you prefer those decisions to any you might have otherwise made. It also helps you to understand that you're allowed to like yourself more than you like other people.

That's a good theory.

You're sneaking in connotations. "Morality" has a much stronger connotation than "things that other people think are bad for me to do." You can't simply define the word to mean something convenient, because the connotations won't go away. Morality is definitely not understood generally to be a social construct. Is that social construct the actual thing many people are in reality imagining when they talk about morality? Quite possibly. But those same people would tend to disagree with you if you made that claim to them; they would say that morality is just doing the right thing, and if society said something different then morality wouldn't change.

Also, the land ownership analogy has no merit. Ownership exists as an explicit social construct, and I can point you to all sorts of evidence in the territory that shows who owns what. Social constructs about morality exist, but morality is not understood to be defined by those constructs. If I say "x is immoral" then I haven't actually told you anything about x. In normal usage I've told you that I think people in general shouldn't do x, but you don't know why I think that unless you know my value system; you shouldn't draw any conclusions about whether you think people should or shouldn't x, other than due to the threat of my retaliation.

"Morality" in general is ill-defined, and often intuitions about it are incoherent. We make much, much better decisions by throwing away the entire concept. Saying "x is morally wrong" or "x is morally right" doesn't have any additional effect on our actions, once we've run the best preference algorithms we have over them. Every single bit of information contained in "morally right/wrong" is also contained in our other decision algorithms, often in a more accurate form. It's not even a useful shorthand; getting a concrete right/wrong value, or even a value along the scale, is not a well-defined operation, and thus the output does not have a consistent effect on our actions.

Things encoded in human brains are part of the territory; but this does not mean that anything we imagine is in the territory in any other sense. "Should" is not an operator that has any useful reference in the territory, even within human minds. It is confused, in the moral sense of "should" at least. Telling anyone "you shouldn't do that" when what you really mean is "I want you to stop doing that" isn't productive. If they want to do it then they don't care what they "should" or "shouldn't" do unless you can explain to them why they in fact do or don't want to do that thing. In the sense that "should do x" means "on reflection would prefer to do x" it is useful. The farther you move from that, the less useful it becomes.