"Its success was rooted in the freshness
of Calvin — an imaginative 6-year-old who has the immaturity of a
child and the psychological complexity of a 40-year-old. As for Hobbes,
the device of Calvin viewing him as alive and everybody else seeing him
as a stuffed animal was simply brilliant, Salem says.
Their all-encompassing bond of friendship — being able to share
joy, yet get angry awith one another — was another reason for the
strip’s success." via Miami Herald.