Many of the most famous novels are either romances or have an element of romance in them.
Most of us have experienced romance which novelists can draw on when writing about the subject. Whatever we write the characters must spring to life from the page and, if the reader is not going to lose faith in them, their circumstances and behaviour be believable.
Consider Pride and Prejudice which contains the classic ingredients of an engrossing romance with many obstacles which inhibit it until the happy ever after ending. When the couple first meet they should not fall in love at first sight, although they can be attracted to each other, on the other hand they could be angry or suspicious, be reluctantly drawn to each other, be wary, embarrassed or annoyed.
The setting must be romantic - a holiday in a five* hotel or cottage with rambling roses round the door, an exotic place - a houseboat in Kashmir, a log cabin by a lake in Canada, Venice, a beautiful island.
When writing about the location, descriptions and language needs to be sensuous, employing the five senses, and conveying food, perfumes etc. Everything should be larger than life.
Romance has it's own language to draw the reader deeper and deeper into the story. Short sharp sentences emphasise conflict and drama. Longer sentences allow the reader 'to smell the roses'.
Historical romance requires extensive research - a foolish mistake will cause the reader to lose faith in the author.
Finally, the reader should be able to fall in love with the hero or heroine in spite of their faults. After all, in Margaret Mitchell's Gone With the Wind Rhett Butler understood Scarlet's flaws and loved her passionately in spite of them.