As josephusrex noted, Calvin differs from Charlie Brown in not feeling alone in a hostile world but wishing he were. This difference comes into play whenever he's more or less forced to play team sports; his anarchic nature, stubbornness and tendency to miss the point of most athletic endeavors make him something of an outcast. We also have to factor in his extreme reluctance to admit when he's screwed up and that there's truth to the saying "To get along, you have to go along." He thus rejects our reality and substitutes his own by playing Calvinball; its appeal is that there aren't any rules to disobey and that he can avoid hearing the character-building words "You're a loser, Calvin Noname." The similarity he shares with the Round-Headed Kid is that while you can feel sorry for them, they also both drive you nuts. The one is a whiny little pill while the other a loud, disruptive nuisance who doesn't want to settle the Hell down.