Imaginary friends may be considered normal during childhood. The level of interaction Calvin has with his imaginary friend is extraordinary and if this was a real life situation, it might be comparable to how Wilson came to be in the movie Cast Away.

I am absolutely not a shrink, but times of extreme stress or some kind of deep trauma might be a trigger to create an imaginary friend such as Hobbes or Wilson. It could he helpful to a person in some circumstances as a means of survival, I suppose.

As a cartoon, Calvin and Hobbes works perfectly. If it were real life, this would be a different situation. It's that dichotomy that may make some of the C&H cartoons seem very strange or morbid to some people.