No one "decides" what is rational and what is not. Rational has a specific meaning, namely, that reason is mankinds only means to discovering and understanding truth. Rational means using reason as the means of determining what is true or false, or right and wrong.
I will assume you really do not know how "whatever a person is content with" is different from and not the same as "rational self-interest," and are sincere in your question. What someone is "content" with means what ever they "feel OK with," with is subjectivism, and not based on reason. Rational self-interest first discovers what kind of being one is, what is good for that kind of being, and then chooses that rationally understood good, even if one feels horrible about it. It puts reason above immediate pleasure (hedonism) and contentment (subjectivism).
No one decided this. It's what means.
Hank
All well and good, but the sum of your post is to simply define hedonism as irrational, without bothering to actually defend the proposition that hedonism is irrational, nevermind proving it. "Hedonism is irrational because it's irrational" is an answer of sorts, but not a particularly objective or rational answer, insofar as it's not much more than empty tautology.
No one decided this. It's what means.
What you said makes no sense at all, as you'll discover when you try to decipher it.
All you've said is that, to discover what my rational self-interest is, I must first discover what my rational self-interest is. Completely circular.