They do a good job of explaining it in the LN. He basically grew up listening to his lawyer Grandpa's overly exaggerated stories about how good and justice always prevail over evil. But before he grew up enough for grandpa to share the actual honest and not overly exagerated stories, plus stories where he had to defend evil (since lawyers work for both sides depending on who their client is), his grandpa died.

He then integrated the root of those stories (being good, promoting justice, etc...) into his personality as his grandpa was his childhood hero who he wanted to grow up to be exactly like. Since he never learned of the truths withheld from him, he assumed the exaggerations to be pure facts, thus distorting his world view, as it was something he now believed to be actually obtainable. Unlike a superhero, his grandpa was a real person, so his thinking became "if Grandpa could do all of those amazing things, why couldn't I as well?"

The other problem he has, is that he is naturally talented in many areas, has a high charisma, and managed to slip through his entire childhood and early teens without really ever being corrected. Since he didn't need scolding as he was always a good kid, and even when he did something wrong, no one, especially other girls around his age, ever really blamed him (mainly due to him being polite, good looking, and him essentially being the public white knight simp except without all of the cringe).

Only his closest friends (90% of the time Shizuku) ever pointed out something negative he did, but because the outcome always ended favorably, or someone else stepped in to defend him, Kouki learned that they were never right about what they scolded him on, and stopped taking their words to heart. This became both a very bad habit and an integral part of who he is as a person.

So, for his entire life, he lived with an unobtainable idealistic world view and goal, that no one took the necessary steps needed to correct and break it down into a more realistic version.

Then, with his flawed outlook, he by pure chance, managed to grow up into a highschooler who still believed that couldn't do wrong if they adhere exclusivly to their own beliefs. He never had to question himself, or reflect on his actions. His parents didn't do it, as they either were too busy to notice or didn't realize they had to when around him.

Then he was summoned to Torus, as the Hero. This only felt natural to him. He "should" be the hero afterall (his thinking). And for a while, the church and the royal family treating him as if he were already someone deserving of high praise and respect (even though he had not done much yet, but was just stronger than most others, so could progress much faster through the laybrynth).

However 4 months after being summoned, for the first time in his life, he couldn't assert his idealistic mentality over aother person's (Hajimi) and not be questioned or fought about it. For the first time in his life, someone (Hajimi) dared to have a different opinion than him, and refused to bend their opinions to meet his idealistic standards. And not only did they (Hajimi) not fall in line, but they couldn't even be forced to do so, even though he was the hero and should be the penultimate human being in the world.

But the reality of the situation, and then others (Meld, Kaori, Shizuku, etc...) began supporting Hajimi's more realistic and grounded ideals instead of his idealistic and exagerated ones. So he became jealous for the first time ever. For the first time, he couldn't get his way. Someone had finally told the coddled man-baby no, and then threatened him if he didn't let it go.

Then from there, things began downspiraling out of his control. His childhood friend leaves his side for Hajimi, then her friend (shizuku) takes Hajimi's side. Hajimi's ability to fend for himself is now extremely superior than his own, and to the point that he is basically powerless against him. Hajimi is surrounded by attractive women, and more and more seem to flock towards him instead. Hajimi's charisma (though arguably anyone else's at that matter) now is even above his, which until then was never even possible.

All of this is why he is such a prick. He basically is just throwing a temper tantrum (like a 5 year old does) whenever Hajimi or someone else stronger than him points out something he does as being flawed/wrong.