The last Valentines gag played out as usual in 1999, with Charlie Brown doing his typical moaning about how nobody loves him (he has already acknowledged years earlier that Peppermnt Patty and Marcie have crushes on him to Linus) typical of Charlie Brown's incessant victim mentality.
The catch here is that just five days after the date of February 15, on February 22 Patty calls up Chuck to ask about her lack of a Valentine from Chuck (pic related), obvious irony here intended from Schulz.
What can be gather from this last romantic interaction from Peppermint Patty and Chuck? That each are obviously responsible for their own misfortunes.
Charlie Brown, despite having full knowledge that he is seen by not one, but two girls romantically still feels the need to complain. A mentality that can only be seen as neurotic or needlessly masochistic. When Patty calls him up, he deliberately dodges any possibility of acknowledgement, in what has now become a running gag.
Peppermint Patty, for her part, is expecting reciprocation on Chuck's part despite doing no such on her own part (we know that she had not sent him any valentines from the strip 5 days prior). Similar to Charlie Brown, this is due to an inherent insecurity. She loves Chuck but doesn't trust herself enough to confidently voice her feelings. She is constantly frustrated by Chuck's inability to understand her feelings, even when she is more direct (we know that Charlie actually *does* fully understand how Peppermint feels about her, he's just his own basket-case, as already discussed). The difference between Marcie and Peppermint Patty is that Marcie is able to grasp Charlie Brown's sensitivity and thus the inherent nature of "Charles" (as demonstrated by the baseball mascot arc of the strip). The irony lying in Patty's frustration at Chuck due to a failure to understand her, while at the same time she's failed to understand Chuck.
They are perfect together as complete blockheads.