Quote Originally Posted by Pravit
Влюбиться means "to fall in love with someone". You don't want to say they fell in love with someone else, do you?
Propp, I never knew that this verb worked in such a way. In English if you say "they fell in love" it is clear that they fell in love with each other. What if you said "Они влюбились друг в друга?"
А, ну да. It is possible to say "Они влюбились друг в друга". There must be an object. But this implies "all-of-a-sudden"-ness. "Они полюбили друг друга" implies "for ever" or at least for a long term.