When Peppermint Patty was first introduced its heavily implied, like in your pic, that *Charlie* actually had a crush on Peppermint Patty (hence the sigh at the end). This idea is seen in an earlier comic as well,after he first meets her (pic related). Charlie eventually grows out of this though.
Why is this?
I believe it's due to Charlie eventually recognizing Peppermint Patty, unlike all of his other crushes as something attainable, as he does realize that she likes him. If there's something we all know about Charlie Brown, its that he cant stop kicking that football. He will continuously subject himself to internal torment because he has a certain comfort in his negative state of being. It's a self-fulfilling prophecy with Chuck. Though it doesn't help that Peppermint Patty acts kinda insensitive towards him at times, seemingly on accident, like asking Charlie Brown to become the mascot for her baseball team, something anybody else would see is obviously degrading.