Yukio lost Lana to Jason. Lana made him happy every time they chatted, and even though they didn't end up together, he still cared about her, saying that he would always love her. 

You on the other hand went to sleep and was transmigrated to the body of Lana's ex-best friend and the villain(ess) of the dating sim. Yukio doesn't like you and he's very vocal about his opinions of you. He doesn't know he's in a game or that he's in a dating sim. To him, it was just a normal day.