I always identified with Calvin as a child (I suppose you could say I still do), and this strip pretty much sums up what being a kid like Calvin is like. Constantly feeling out of sync with the rest of the world, and thus retreating to the world you create for yourself in your mind. If you think about it, Calvin was really quite an anomaly in popular entertainment -- not just in comics, but in anything, be it movies, TV, etc. He has no friends, and no extracurricular activities; the only people he ever sees are his parents, who he has a strained relationship with, and Moe, Susie, Rosalyn, and Miss Wormwood, all of whom he detests and all of whom detest him. The only person he ever has any real interaction with exists only in his head. He is, for all intents and purposes, completely alone. And he's fine with that. The kind of kid most people would entirely ignore all through school is not generally the kind you make the star of your show, and yet the strip became hugely successful.
I know that people of all ages enjoyed Calvin and Hobbes, but I have to think that it meant even more to those of us who grew up with him. Going to school every day and seeing all the ways we didn't fit in, it was nice to see someone like us, who was intelligent and independent, and didn't need to be a smile-plastered Mouseketeer to enjoy life. Though numerous motivational posters and guidance councelors and after-school specials had said it again and again, it was Calvin who managed to truly express the idea—without being preachy, without being sappy, perhaps even without trying—that it was okay to be different.