People react to things. And they react to most things with dialogue. I like to let my characters conversations work around their reactions. Something happens-conversation. And you know what the fun thing is about dialogue? It leads me to different places. New places. Places I never dreamed the story was going. Who was it that said dramatic dialogue has two audiences? I think it was Aristotle. The character being spoken to and the reader. And the writer is talking to both. Pretty good stuff if you ask me.
Writers need to listen to the world around them. Writers need to listen to their characters. I've read books where I knew the author never heard his characters speak. He typed what he thought they would say. We know that talking to your characters is a good thing to do. Don't we become our characters when they talk? I was Anna in my MG story. At times I was Claire too. They told me things. They showed me things I never expected them to.
An exercise I learned some time ago. I forget where. Strip one of your existing dialogues, so much that you take out the original scene. Ask someone to create a list of four or five details from your world. He might choose a detail from several different settings. He might use senses. Have him write this on an index card. Tape it to your computer. Rewrite the original dialogue and use these details.You might have to reshape some other things like the setting, etc.. But work in the original dialogue as much as possible. The part where you stripped it down. Allow the dialogue to react to the new details. Change context and speaker. When you finish a paragraph, if you don't see a scene, start over. Do this four times. Read them and look at what has happened. This exercise can take your story in directions you never knew it could go. And it can also lead to a new story. One that you never even thought you would write.
You'll be surprised at what you find. :)