Totoro is the God of Death
This theory says that Totoro is the God of Death, so the persons that can see Totoro are actually close to death, or already dead. What that means for the story is that when Mei goes missing and a sandal is found in the pond, Mei actually drowned. When Satsuki is asked about the sandal she cannot face the truth and lies about it not being Mei’s sandal. So Satsuki goes on a desperate search for Totoro, calling for him and actually opens up the door the realm of the dead herself. With Totoro’s help she finds her dead sister and they together go to their mother’s hospital. There, the only one who actually noticed that the sisters were there, was the mother, who also soon is going to die.And in the ending scene, Satsuki and Mei don’t have any shadows some says.
The storytakes place in Sayama hills where also Hachikokuyama Hospital is, which is believed to be the model of the hospital where the mother is Shichikokuyama Hospital.
There is also a murder-case, the Sayama incident, which has many scary coincidents with Tonari no Totoro. Especially that it all happened in May and both of the sisters names actually are May – Satsuki, the name of the older sister that means May in Japanese and Mei, the name of the younger sister, which is how Japanese people pronounce May.
Furthermore, the fact that the little sister went missing and that both sisters later turned up dead. According to some rumors, the big sister saw her little sister’s corpse and went crazy and later should have mumbled something alike “I saw a ghost/monster cat” and “I met a big raccoon” = Totoro, before committing suicide.