(SPOILER ALERT)
I was delighted to see this subtle/multi-layered tale unfold, which seemed to play well to both a childhood audience, keen on likable characters/compelling imagination, and adults with a deeper appreciation for the symbolic backstory.
Like most of Miyazaki's films, there was an enormous focus on the power of the natural world. Whenever man perverts the natural world, the natural world seems to pushback. It appears as though the seldom seen Mother in the story is sick due to some mental/physical disconnect that she has likely had with the natural world.
Through the unique power of children to tap into their imagination, especially children living outside the city and amongst nature, Miyazaki demonstrates the unique gift that children have to re-establish the lost bond between unhealthy adults and the healing power of the natural world. I was thoroughly impressed with how delicately Miyazaki established this bond between the two children, their imagination, and the natural world. Totoro, the lovable silent giant, is a great and unexpected metaphor for the healing power of the natural world and childhood imagination :)