I did have the interest to read the WB or LN on NU, but the description kinda made it hard for me to know what exact type of novel it was because FOR ME the description in there felt more like he was the son of the hero but he didn't want to be the hero but everyone was forcing him to be one, then he found the Demon lord and was forced then willingly accepted to become his disciple, just because he wanted to be something else, this is just what I interpret from that description
While here the description felt more in line with what I've read here, and even from only reading these three chapters, I could grasp that he's like that due to lack of parental support, unequal comparison, heavy expectations, and honestly lack of help. Because from only reading these 3 chapters, it's easy to see from the conversations that even when he has good grades he's mocked or has his standing lowered due to being compared to someone who's 'better', the ' ' is because Fiancei is better than him because he was never properly trained and trained alone at something he's not suited at something that was probably different for her, from again the conversations it's heavily implied that they want him to be a sword hero and lowering him for not being good at the sword, and his parents that were never by his side also end up causing him to lower himself just see from how his father, the hero, said that the MC should've got 1st place because he was his son and the only one who accepted him by what he was was the maid, but even she was too passive at what she did to help him, thus only causing his eventual demise
I would say it's kinda like you being good at a certain school subject, let's say history, but everyone compares you to someone who's better than you in everything, such as math, then your parents are always away so they don't support you and maybe even say that you should be better at that subject you simply can't get good enough no matter how hard you try, and in the case of this manga, the only person you knew from childhood suddenly began to badmouth you from behind your back and in front of you, while the only person who supports you don't take a more active role at helping you. Then all of a sudden someone comes in says what you have to do, teaches you properly and shows you that you must do what you want and what you're good at, even if that end up making you distance yourself away from your family
That's my personal take at what I understood from the manga until now, and from some comments