For those who didn't understand, Soyeon (the girl he's talking to) told Jaehee that the reason Dojun didn't accept his admission based on the director's recommendation had given him was to leave a place for someone else, and Jaehee didn't know that he did that so Jaehee, who had come in third place, had a place to be adimitted, he discovered that in this chapter. (I had to read the version in my language just cuz I wanted to help you lol)
To sum up, Dojun left Jaehee for Jaehee's own sake
