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.