Okay, don't get me started on the cursed lycan trope because I will actually never stop. What works for me is the inherent friction between duty and desire. It's not just a shifter romance; it's a character who is literally at war with themselves. Their 'curse' is a monster inside, a beast that might hurt the very person they're biologically or magically compelled to protect and love. That push-pull creates unbearable, delicious tension.
You get scenes where the protagonist is fighting the shift, maybe their eyes are glowing, claws are extending, but they're holding onto their human consciousness because their mate is right there, vulnerable. It's the ultimate test of willpower. The 'curse' often ties into pack politics or ancient family sins, adding a gothic layer of doomed legacy that they have to overcome, which is so much more compelling than a simple fated mates story where everything is easy. The struggle is the whole point, making the eventual surrender or mastery of the curse feel earned. That catharsis is unmatched.
Authors like Lola Glass or Sarah J. Maas (with the Illyrians in her work, sort of an adjacent vibe) really play with this duality, making the internal battle feel just as high-stakes as any external villain.