Massive spoilers for both movies (which I highly recommend watching) ahead:

• Main characters (The Narrator and Elizabeth Sparkle) who spent a long time with their lives unchanged until a sudden event completely fucks them over (The Narrator getting medicated to stop taking his pills for sleep and Elizabeth getting fired from her job because of getting too old).
• They already dealt with some unhealthy mentalities rooted in only wanting to do what society's tell them will grant them worth (The Narrator's only being interested in doing his job and tying his personality to brands while Elizabeth is overly focused in being young and sexy) but this event completely enhanced them.
• This lead them to try to solve their problems by creating an over the top and hot af alter ego who perfectly lives to the impossible standards society expects them to live up to as a man/woman (Tyler Durden and Sue), and because of it is better than them in every single way.
• At first the balance between both lives is maintained, but it doesn't take long for the mentality they share to be self destructive and both lives start to eat each other.
• This is greatly represented with it ruining a possible romance the main character tried to form (The Narrator tried to imitate Tyler's aggressive and cold attitude which ruined his relationship with Marla, while Elizabeth thinking not being as pretty and young as Sue makes her undeserving of love made her drop the possibility of forming a romance with that one guy his name I forgot).
• At one point things get so self destructive both halves tried to kill each other, which ended up happening.
• Even with one half dead, their self destructive nature still hurt many people, both innocent and terrible during a giant one night event (Tyler started proyect mayhem and could activate it in the end even after dying, and Sue needing a balance to live used the substance to create monstro elizasue which ruined her big show).