The thing that new writers often don't understand about dialogue is that it's not really how people talk. It's how we would like them to talk (especially in those romantic or heartfelt scenes). Real life dialogue is full of umms, and ahhhs. It's full of repetition and shorthand. Much of it doesn't even stay on track. The conversation meanders through many half conversations. People get interrupted and never return to the topic.
Fiction dialogue, be it in plays, movies, or prose, needs to do what all writing needs to do - push the plot forward or show us a new aspect of character. It has to be directed by the speaking character's goal and motive. Why is the character saying this? What do they expect to get out of this conversation? How is what the other character or characters saying impacting what is said? And even more importantly, what is being left unsaid?
Dialogue in fiction is duel - even if the two parties are friendly. Each person wants to achieve something and that want will create the conflict you need in your scene to push the plot forward. Real life conversations are only sometimes a duel. A good deal of the time neither party really wants anything other than to have a conversation. At most they might want to look good. The really juicy conversations are the ones that end with a phone call to someone else to vent about how the person you were just talking always seems to want something.Every dialogue scene in fiction should be that kind of conversation.
Dialogue without purpose slows pacing to and dilutes the plot. So write tight and make every word in dialogue count. Make every word a highly calculated jab, dodge, or weave and you will have dialogue that your readers will praise for being so "real".
Next time - Dialogue tags.