There's so much sadness in Pelican Town it's almost unfair to say with certainty, "[this] is the saddest," but I think it's super sad that you can't foster a friendship with Marlon, Gil, Gunther, or Professor Snail; cave divers and bone collectors need companionship too! And speaking of Marlon, isn't it sad that Marnie could have that stud but chooses to continue seeing Lewis in secrecy because he's soOoOooO concerned about professional appearances? Marlon thinks you're pretty, Marnie, he says it every year at the Flower Dance, how often does ol' Lewis show genuine affection? AND they would have a cute couple nickname: MarMar is a heck of a lot better than Lewnie, but one would certainly have to be "lewnie" to entertain an affair with a mayor who commissions a solid gold statue of himself while verbally lamenting the decaying community center. She frustrates me because her schedule drives me mad, but she can definitely do better.
I also wish you could befriend Birdie, but I respect her choice to live in solitude by the sea 'til she's reunited with her love in the afterlife because I'd honestly do the same. This may have been mentioned somewhere on the forums before, but do any of you think Birdie's husband could be the Old Mariner who you buy the Mermaid Pendant from at the tidepools? It's so tragically adorable he uses his ghostly powers to stoke romance in the valley if that's the case.