He is innocent and ignorant, like a blank sheet of paper
She is gentle and sensible and treats him like a brother
She taught him a lot. She always touched his head and called him "fool" with a smile.
When he changed from a blank slate to a sophisticated person, he looked back and realized that she was just one of thousands of women in his heart.
Many years later, he got a new girl. She was simple and confused and always made mistakes. He silently helped her clean up, then touched her head and laughed and called her "fool".
Only then did he realize that he was still the little fool he was back then.