BL manhwa refers to Boys' Love comics or graphic novels that come from South Korea and feature romantic or intimate relationships between male characters.
There were indeed some similarities between Sunny Days and Days Relating to Youth. For example, both stories described the growth of young people in puberty, encountering various difficulties and challenges, and overcoming them through hard work and courage. In addition, the two stories also depicted topics such as friendship, love, and dreams during their youth. However, there were also many differences between them. For example,"A Sunny Day" described the cultural background and social environment of China at that time, as well as the emotional entanglement and growth process between the two protagonists, while "Days of Youth" paid more attention to the study life, interpersonal relationships, career planning, etc. of young people in the United States.
Well, a rut bl manhwa usually features male-male romantic relationships and explores various themes related to love and emotions. It's a popular genre in the world of manga.
The lyrics " Your sunny day is my rainy day " came from Eason Chan's song " Sunny Day." This song was a congratulatory song for Eason Chan's wedding. It was composed by Xu Changde and Chen Jianning. The song was about a cruel comedy. The protagonist's life was left with someone else, but he still had to pretend to be cool and attend the wedding. This lyric expressed the pain and helplessness of the protagonist who had lost his lover.