Reincarnation genre Manga is popular in Japan and not just because of the spiritual themes. It's also popular because of its interesting characters, intense emotions and comedic moments.
This genre has been adapted into many different forms of media such as anime, video games, movies and novels. There are many types of reincarnation manga. The most common one is a love story with a mysterious person who saves the protagonist's life by turning them into their younger self. While many people like this genre due to its lightheartedness. It can also be depressing at times depending on how serious it gets.