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.)
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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). Somewhat younger 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 into the metaphorical sunset, Marshall's arm around Simon's shoulder.
My Secret Identity (tv show, CTV [Canada], 1988-1991)
"Best Friends" (21 May 1990)
Andrew (Jerry O'Connell) and Kirk (Christopher Bolton) have been best friends since
the beginning of this season (2), and friends since their early school days, interrupted
for a few years when Kirk's family broke up and he moved away for a time. This episode
begins with the teens bantering, and battering baseballs in a batting cage. (Naturally,
their friendship is embodied for the show in sports.) Their easygoing closeness is clear.
Without much warning (this show typically moves too fast for its half-hour time slot),
Kirk is unavailable to Andrew either for play or work because of a relationship Kirk
is pursuing with a girl. Kirk makes it clear that everyone knows a romantic
relationship always takes precedence over fun and work with pals. Andrew is disappointed,
to say the least, in the usual way these things are portrayed. Spoiler alert: by the end
of the episode, Kirk fumbles with the girl, apologises to Andrew, and vows such things
will never come between them again - as Andrew is going out with his own new girlfriend.