First, the format for a romance novel often requires a relatable protagonist. Readers need to be able to root for them. Second, the relationship development should be believable. It can't be too rushed. There should be shared experiences that bring the couple closer, like going through a difficult time together. Also, the use of sensory details is important. Describing how a character feels when they touch or how a place smells can enhance the romantic mood.
Well, in the format for a romance novel, one key is the emotional arc. The characters should start with some sort of attraction, then face obstacles that test their feelings. Another aspect is the dialogue. It should be both flirty and revealing of their inner thoughts. And don't forget about the build - up to the big romantic moment, like a first kiss or a declaration of love. It should be slow and suspenseful.