Designing romanceable NPCs sounds a lot easier than it actually is. But that is not to say that it is difficult. It just takes a little more time, patience, and a player character to romance.
Prior to determining if an NPC will become a romantic interest for a character, start by determining what role the NPC will have in the story if the PC never enters their life or never tries to romance them. Then you can write a backstory that compliments that version of the story. Are they a barmaid, a quest giver, an on-going ally, or maybe they are the games penultimate BBEG? Strive for giving them a life of their own, outside of being a romantic attachment for the player character.
After giving your NPC a life of their own, with their own likes & dislikes, now it is time to get dirty and do some digging. With any luck, your players have given you in-depth backstories their characters . . . if not, send them to RPG Storytelling and we'll help them create one. Start by reading through those character backstories and try picking out individual pieces that might provide some kinds of connection between the NPC and the PC. Some things you could looks for are:
Both are from the same town,
They both like the same colors,
Both are from the same character class (e.g., barbarian, cleric, rogue, druid, etc.)
They both like to fight (despite character class)
They both enjoy the same weird combinations of foods (e.g., terrasque steaks topped with gelatinous cube gravy).
Once you uncover a few juicy pieces of connectable information from one or more of the PCs, start incorporating them into your NPCs backstory so that when the PC meets this person, they start to develop a friendship, and over time, perhaps even a romantic relationship.
But the fun really comes into play when you realize that while some NPC / PC relationships are destined to last forever . . . others aren't. Even though it worked for Donkey and the Dragon in the movie Shrek (2001), the likelihood of a Gnome Bard and an ancient red dragon surviving the test of time is, well, shaky at best. You just know that the dragon is gonna get hungry in the middle of the night and Gnomes sure do make for some rather tasty bite-sized snacks.
While some romances happen on the fly (as through spellcraft), others will probably develop over time. Which means that your NPC is going to need things to talk about when your PC comes home from adventuring. Which in turn means that that NPC is going to need a bit of a backstory to help keep the romance alive.