In The Lord of the Rings: The Rings of Power, viewers are introduced to a conversation between Amazon's original character and his friend, in which the latter discusses the tragic history of human friendship. In Arondir's eyes, Arondir is foolish for allowing himself to fall for Bronwyn, a human healer from the nearby town of Tirharad (also invented for the series).
The point is, he's sort of exaggerating. While there arent many examples of relationships between Elves and humans (at this point in the timeline, there have been precisely two), to categorically state that they ended badly is a bit ludicrous. However, ultimately, what's tragic and worthwhile?
The most famous example of a human falling in love with an elf in The Lord of the Rings is, naturally, the tale of Aragorn and Arwen: If you want to marry her, you must become the King of Gondor and Arnor, Aragorn does this after the War of the Ring, and Elrond returns home to Valinor never to see his beloved daughter again.
That last part is a bit sad, but given that Arwen chooses a mortal life that is something well-thought out later on, she and Aragorn get to spend the rest of their lives together in Middle-earth, which is actually quite lovely. Anyway, that happens long after the events of The Rings of Power. The two relationships that Arondirs pal is referring to are those between Beren and Luthien and Tuor and Idril.
Beren and Luthien
The first encounter between humans and elves occurs in the First Age, when Beren, the mortal man, meets Luthien, the elven princess in the legendarium, having been written just one year after Tolkien returned from World War I almost four decades before The Fellowship of the Ring would be published for the first time.
Beren and Luthien is a vital story, that it was posthumously expanded into an entire book, organized by Tolkiens son and dedicated editor Christopher. Beren is given the difficult task of retrieving a Silmaril from Morgoth (if you thought Sauron was bad, this guy used to be his boss).
Despite failing to retrieve the Silmaril, they return to Doriath and are given a hero's welcome. Years later, the wolf Carcharoth in the service of Morgoth himself travels to Doriath, at which point Beren sets out with a party to complete the Quest of the Silmaril.
Beren dies, but his spirit waits for Luthien in the Halls of Mandos. She does some weird spirit stuff and somehow gets an audience with the Valar Manwe, who gives her news of her predicament to Eru Iluvatar, literally God. At this point, she chooses the latter option, becoming the first elf to die of old age in Middle-earth.
Beren and Luthien meet up after becoming heroes, and they manage to find time to quietly live out the rest of their years together, according to Arondirs friend. But it's not really that bad.
Tuor and Idril
In the First Age, the second elves-human relationship comes about, this time centered on Tuor and Idril, who face considerably less judgment than Beren and Luthien, to the extent that Idrils father, who is also a king, sees Tuor as a son. He just says, I'd like for you to marry my daughter.
Except Maeglin, a weirdo who was so obsessed with Idril and jealous of Tuor that he sold the whole city out to Morgoth, instigator of the Fall of Gondolin (which was later expanded into a whole book by Christopher Tolkien).
Although there are other technically related connections between humans and elves, these are not strictly between Edain and Eldar, which is to say, humans and elves. The majority of these other pairings are between half-elves, or peredhil, or Tuor and Idril (in fact, Arwen is technically peredhil), which is why Arwen stays in Middle-earth while her father, Elrond, returns home to live forever.
The whole reason for their love for a historically significant city angle could be construed as It ended badly if their story ends there. But it doesnt. They escape Gondolin with all of the survivors and rebuild their society at Sirion. They even have a child: Earendil, who will grow up to be a prominent figure in the legendarium. Eventually, they leave Middle-earth and go west to Valinor, where Tuor, who is still loved by the elves, becomes
Wow, what a terrible experience. Iluvatar forbid that Arondir and Bronwyn would be treated differently.
When it comes to The Rings of Power, I mean, the odds are pretty good between Arondir and Bronwyn! The other two couples ended up being extremely content together. In my opinion, Arondir's friend is just a bit concerned about the possibility that they won't get to hang out as much anymore. What a terrible friend.
All of this being said, the fact that Arondirs pal does anything about it is a bit foreboding. Misfortune is likely to be on the way, especially when you consider the very Nazgul-y sword Theo, Bronwyns son, discovers early in the season.
Yes, this one will most likely go badly, but not because of the ones that preceded it.