Anybody who knows Peanuts knows that Charlie Brown is forever pining after the Little Redhead Girl, an unseen character who he never works up the courage to speak with. The earliest years of the strip, however, featured Charlie Brown in an on-again, off-again relationship with another character - a girl named Violet, who was the first major addition to the initial cast after the strip's start. 1953 would give her the surname Gray, a move I can only assume was intended to underscore a parallel between her and Charlie Brown by giving them both color-related last names. (Mind you, it's only ever mentioned once, but feels significant to me in light of very few recurring characters in Peanuts actually having surnames.)
Throughout the rest of the 1950s, the two became less and less friendly. I can't imagine there was any kind of real continuity intended by Schulz, but reading the strips in The Complete Peanuts in sequence as I have been, it becomes painfully apparent that Charlie Brown becomes more desperate to have Violet's approval - to have her as a friend - as time wore on. In a vacuum, you wouldn't really think anything of the individual strips, but looking for whenever the series came back to just the two of them... you feel like you're seeing their relations fall apart piece by piece, like a seaside cliff as it is slowly eroded by the waves of the pounding surf. It might be one of the most subtly depressing things I've seen in comic strips.
(The pink panels come from separate strips - in chronological order, IIRC, but I didn't think to save the dates - while the purple ones are a complete strip from 1952.)
They're playing our song...