Ji-Seok was never a particularly brilliant person in anything he did. He had a good physique which allowed him to play rugby and basketball his entire life but he was never the best at them. He learned how to fight but would still end up being defeated by stronger, more skilled people.
Seeing those people achieving great things while he always fell behind frustrated him greatly and made him feel inferior. He knew that he put a lot of effort into everything he tried and yet that didn't seem to be enough to make him the best at something.
So, when the Integration happened and he miraculously killed his first few Dimension Walkers, he saw the others around him being impressed by that achievement. For the first time in his life, he saw people's eyes on him.
He felt really special, but also, started craving that attention more than ever. Wherever he went, he wanted the others to talk about him and hail him as one of the greats.