In a way Calvin was growing up and becoming a more introspective and thereby pessimistic person growing into young adulthood, leaving his carefree days behind.

In a way Calvin was growing up and becoming a more introspective and thereby pessimistic person growing into young adulthood, leaving his carefree days behind. At least that is what I took from it. Then the last panel, if I remember correctly, was an ode to the carefree days of childhood.