While the subject matter is definitely for mature audiences, it has gained a following for a few specific reasons:
The magic usually lies in the contrast. You have an adult who has forgotten how to have fun, paired with a child who sees the world with wide-eyed wonder. Watching a protagonist learn to cook a proper meal or navigate a playground for the first time creates a perfect blend of comedy and character growth. 2. Low Stakes, High Emotion shinseki no ko to o tomari dakara de na fe