So, they're human beings.Personally, I think people who insist on confessing to someone who they know already has a significant other are among the worst sorts of inconsiderate, selfish and self-centered individuals.
So, still human.They think only of themselves and do not consider or care what adverse affects their selfish actions might cause to the object of their affection ("Eiji getting completely flabbergasted"), or to the significant other ("Nanako getting scared out of her wits").
Homo sapiens.They cause others to suffer emotional pain and distress just so they can feel good that they were able to express their feelings ("Iori running off with a smile now that she feels lighter").
Welcome to humanity, enjoy your stay if you can. If not, well, it happens.What a bunch of hogwash.