save vs. True Love

In those days people fell in love much more suddenly than at present, as all ancient stories make manifest: it is not a matter of wonder, therefore, that the hearts of the three cavaliers were completely captured; especially as gratitude was added to their admiration; it is a little singular, however, though no less certain, that each of them was enraptured with a [a different one of the three princesses].

- Washington Irving, Tales of the Alhambra

Being struck by love in the old stories has a few common features:

•  It's "love at first sight," with criteria that may not be entirely shallow - an intuitive sense of the goodness or innocence of the beloved isn't a contributing factor - but definitely include beauty and exclude, say, a shared history of mutual respect and compatible goals
• There's definitely not a presumption that it's reciprocated; that's for you to figure out, kid. The map of lovers and beloved is neither surjective nor injective and certainly not bjective.
  
• Despite the shallowness of its causes, it's not shallow in its effects; you want your beloved to love you back forever. Any basically decent person will want to enter into a mutually happy and permanent marriage with their beloved. Evil tyrants and other bastards with no sense of mutuality will kidnap and abuse the victim of their infatuation, but even they will be frustrated if the beloved  withholds (as they typically do) the assent of their heart even as their body is stolen. 
• There's an unlimited range of language springing from a very limited range of metaphors: to be struck by love is to be enchanted, bewitched, or completed; to be with the beloved is to have the greatest treasure; to be without is to be sick or struck dead.  

Here are a few OSE-comptaible rules, easily adapted to similar systems:

1. When you encounter a potential love, save vs. Spells with advantage to avoid becoming Lovestruck. "Potential" here takes account of age, species, and if appropriate gender; it definitely requires (1) that you not already be Lovestruck and (2) that this is the most beautiful potential love that you've ever seen (or, if you hear them singing unseen, heard). (Hence why the young are so much more likely to be lovestruck than the old, though the latter is still quite possible.) Players are welcome to set in advance criteria for whom their character might fall in love with, but once this is set, it’s down to the throw of the dice, not convenience to the plot or anyone else.
   
2. Marrying your true love is a treasure worth 1,000 gp, doubling every year that they're alive, faithful, in love with you back, and not imperiled. Them ceasing to be any of these things may result in XP debt.
3. If your beloved becomes definitely not alive, faithful, or in love with you back, save vs. Poison each full moon or suffer the effects of a Cause Disease spell. The death of such a disease is a deep depression that can only be cured, of course, by True Love's Kiss, though mournful elf-song, perhaps, can abate it.

With the consent of the lovestruck person only, being Lovestruck can be obviated with a Dispel Magic or Cure Disease.