Same-sex best friendship is a hallmark of late childhood, for girls and boys alike. Opposite-sex romantic love is a hallmark of adolescence.
The 'break-up' of best friendships in favour of romantic love is a common theme in literature (including television/movies) about adolescence.
The typical 'trajectory' is 1) friendship is portrayed, strong, intimate; 2) the love interest on the part of one friend intrudes, making the other friend feel left out (the word 'jealous' is often used); 3) the original friends make up (whether or not the love interest continues), pledging that in future no love interest will disrupt their friendship ever again. (For boys/men, the phrase "BROs before HOs" is often part of the diaglogue.) ============================================================================
Song: "Wedding Bells (are Breaking Up that Old Gang of Mine)" (1929) barbershop, Sammy Fain (music), Irving Kahal, Willie Raskin (lyrics) https://en.wikipedia.org/wiki/Wedding_Bells_Are_Breaking_Up_That_Old_Gang_of_Mine Television: Even Stevens (tv show, Disney, 2000-2003) "My Best Friend's Girlfriend" (22 March 2002) Twitty (A.J. Trauth) is spending more and more time with Allison (Tania Gunadi) to the exclusion of Louis (Shia LaBeouf), who becomes jealous and temporarily switches his allegiance to Tom (Fred Meyers). "Stevens Manor" (28 October 2002) Ren (Christy Carlson Romano) gets tired of hearing from her friend Ruby (Lauren Frost) about her boyfriend Dax, so when Dax and Ruby break up, Ren arranges a "no boy zone" sleepover at Ruby's house, then gets miffed when Ruby makes up with Dax on the phone and spends an hour and a half talking to him. Eerie, Indiana (tv show, NBC, 1991-1992) "Heart on a Chain" (3 November 1991) New girl Melanie Monroe (Danielle Harris) arrives in Eerie, and when introduced to Marshall's class, gets the attention (to say the least) of Marshall (Omri Katz) and his pal Devon Wilde (Cory Danziger). Simon (Justin Shenkarow) can't understand the new attraction - "But guys . . . she's a girl!" - and feels left out as Devon and Marshall compete for Melanie's attention with gifts and show-off feats of skateboarding and bike-riding. WIthout going into the details of the story (to avoid a spoiler-alert), Marshall and Melanie agree at the end that it's better they not continue their relationship. Conveniently, Simon is in the background of the parting scene, and as Melanie walks away, Marshall sees him: "Hey, buddy, wanna go look for UFO's?" he says, and they walk off, Marshall's arm around Simon's shoulder.