I've always thought dialogue writing was a bit like comedic timing - it either comes naturally or it doesn't come at all. And, to a certain extent, this is true. The very best dialogue writers will always be those to whom the art comes naturally. Like musicians with perfect pitch, some dialogue writers "hear" the sound of the words they write so well, they can tell when they've written just one word too many - or too few. Many writers fill their novels with exposition, in part, because dialogue writing is so terribly difficult for them.
However, with an understanding of the art and craft of dialogue writing and lots and lots of practice, even those with no "ear" for dialogue can master their craft.
One of the first things any writer has to ask himself or herself before writing even one line of dialogue is this: What purpose does dialogue serve in a work of fiction?
In general, dialogue in fiction serves one or more of three main purposes: (1) it advances the plot; (2) it defines character; and (3) it offers exposition.
Read the dialogue you've written very carefully and as objectively as you can. Make sure it conforms to one of the three purposes above. If it doesn't, cut it, because it's going to detract from your work rather than add to it.