That is supposed to be objective.
Why is being a hedonist irrational if one is perfectly content in being one?
It's not irrational, so long as being a hedonist is in your rational self-interest (as opposed to whatever you think your self-interest might be)
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