Let’s imagine for a second that Charlie Brown were to actually meet Calvin; odds are that they wouldn’t get along too well. First off, the Blockhead is fairly tiresome company and would tend to irritate the boy with the is-it-a-toy-or-not tiger with his endless whining about how bad his life is, how nobody likes him, how he alllllways loses, how he can never seem to make any headway in life. Since none of the things that Charlie Brown obsesses about mean a blasted thing to Calvin, he’d probably try to steer the conversation to something that does matter to him. Let’s also not forget that the reason that Calvin finds the sort of obedience and conventionality that the Round-Headed Kid takes as a given so hard to bear is that he’s almost inhumanly intelligent; having to play by the rules is something like agony to him which means the isolation that Charlie Brown thinks he lives in would be a state to envy, not dread or moan about. It would be something approaching Nirvana to live his life in a void wherein no bullies, teachers, parents or any other unwanted company could intrude.
Another reason that they wouldn’t get along is pointed to not only by Charlie Brown’s non-stop pessimism but by the fact that Calvin is very much his parents’ child; he draws his keen mind from his father and his imagination from his mother so it’s clear that sooner or later, his intellect would be recognized for what it is and he’d be able to channel it into the same sort of constructive channel as his former pain-in-the-ass-as-a-child dad; the Blockhead is on a bullet train to mediocrity and knows it and would then tend to resent anyone who’ll outshine him.