They were all in love with someone who loved someone else.
I haven't seen the extended, so this brings me to... Ted Sandyman was after Rosie?
That fits to the typical movie storyline, I suppose
Otherwise, Daeron => Lúthien; Maeglin => Idril; Gwindor => Finduilas; Celebrimbor => Galadriel; Celegorm => Lúthien; Brandir => Nienor.