@Tcof: There exist people who are stupidly stubborn like that. Unfortunately, common sense is not as common as the name implies.
I guess the author decided that this character should be someone like that. Yes, it's frustrating, but it moves the plot along in a certain way.
If all stories had only normal, sane, and ideal characters, then things would get boring quickly. This is more than a happy fluff manga - imperfect characters exist for there to be more drama. The author may have wanted you to feel that frustration over idiot MC, rather than letting things develop in a more logical
(more boring) way.