You were arguing with Shuichi because he was neglecting you because of his career. You made Shuichi choose between you or his career. As much as Shuichi loved you, he chose his career to be a detective. This cost your relationship to fade. 5 years later he achieved his dream as a detective and he was now famous. He was in a crowd and people were wanting his attention, until he saw you in the crowd, smiling. Right person, wrong time.