Dialogue is a big topic; too big to cover adequately here, but we can get a good start on the basics. It sounds like you're already paying attention to dialogue in books that you admire. That's a great way to refine your "ear." Make sure you're reading actively. If you liked the rush of a particularly passionate exchange, for example, take a look at what choices create that effect. Here are some other pointers to keep in mind:
Pay attention to real-life speech. Writers can learn a lot about authentic dialogue by listening—and making note of—the way people talk in real life. Even a brief stint of eavesdropping will reveal that people don't speak in the complete sentences used when writing. They also tend to use contractions and colloquialisms. And rarely does anyone say the listener's name aloud. Individuals also have distinctive voices, including hallmark phrases they return to again and again. People use words and make references that are particular to their experience, education, or occupation. Your characters should, too.
Dialogue, however is not a straight transcription of real speech, so you want to distill a conversation down to the essentials to create dialogue. Reading a bunch of false starts, hesitations, and filler words, which are common in real speech, can quickly become tedious. And you don't need to include all of a conversation that would take place in real life. Forgo the little pleasantries and small talk that pepper most conversations and cut to the heart of the exchange.
Stay true to human nature. In real life, people don't say exactly what they mean in precise and clear declarations. Sometimes they approach a difficult or charged topic indirectly because they don't want to talk about it or don't know how to. Even when intending to be direct, people struggle to connect what they feel with words. Things don't come out right. Or they say something they don't mean in the heat of the moment. One technique to try is subtext, where the truth exists underneath what is spoken. A conversation between a mother and daughter about the daughter's failed dinner could really be about the mother's lack of confidence in daughter's ability to make good decisions. This can add dramatic impact in addition to authenticity.
A well-written conversation isn't just about what happens between the quotation marks. Use narrative to inform dialogue. This might come in the form of setting, action, or thought. You can ratchet up the intensity of a conversation between a married couple about the state of their bank account by showing the husband bang the dinner dishes he's washing. Narrative also simulates the passage of time, creating pauses in the dialogue. Don't put too much narrative in between bits of dialogue that would follow closely if the scene were playing out on a stage.
There are also some common mistakes writers make in crafting conversations. One is expository dialogue, which attempts to explain too much to the reader using exposition. For example: "I went to Gobel's Market today. It's that place where we saw that kid get caught for shoplifting. And he was only eight years old. The owner was so mean, holding him down until the police came." This listener was there. She already knows what happened. Instead, use the power of suggestion or narration to convey the information to the reader.
Also, avoid adverbs in your dialogue tags ("she said angrily") or overwrought verbs ("she admonished"). For most tags, use the verb "said" and let the dialogue itself do the work of conveying the emotion or thrust of the delivery.
Keep these issues in mind as you write and revise, and you'll be well on your way to smoothing out dialogue. For more information, check out Leigh Michaels's book On Writing Romance. Her chapter "Writing Dialogue and Introspection" may be just what you're looking for to spruce up the exchanges in your romance novel.