Romance is just another progression step for the character. Romance shouldn't always have to be ooey-gooey, 'I'm in love and we're in love, and the whole world is in love!!!' scenarios.
Romance can lead to the death of a character.
It can be their greatest strength, or their greatest weakness.
Romance can be completely one sided.
Romance can consist of more than one other interest,
and cause tensions between characters.
I mean, w
ho would Daenerys Targaryen be, had she not met the Khal, and all the events that trailed after? Still a scared little girl, under the thumb of her abusive brother.
How would Han Solo have escaped from carbonite, if someone didn't love him enough to set up a rescue mission and save him?
What about Simba without Nala?
Katnis without Peta?
Thor without Jane?
Romance isn't always necessary, but it does open up doors to help us connect more with our characters and even bring in a sense of humanity to them. Romance should add another flavor to the story, not take away from it. Romance takes time in real life, take time with it when writing your characters.
Ultimately, it's another tool for development. Why not use it? Whether or not that's with another PC or NPCd.