As a writer you might just write whatever comes to you whenever it comes to you. If you're writing a novel and you feel you need pages of dialogue then you'll write those pages without regard for whether they are best kept in that novel. When you write dialogue you are translating something from its natural state to an unnatural one. The ideas behind the dialogue are better conveyed in conversation with real people and not ink and paper.

If you find your prose is overloaded with dialogue you might be writing a novel and play together when they actually want to be separate works. You might not have given thought to the expressiveness that is lost when simply letting dialogue be read and not heard. There is a reason certain types of novels get adapted for the stage.

Inflection

It's easy to lose the exact meaning of a line when it's read. You have to make guesses as to which word the emphasis is on. Character don't often say lines just as you would read it with uniform saying of each syllable. These are details that can add texture to the story when performed audibly but can't be as elegantly conveyed in text form.

Pauses, Rhythm

We tend to read in a monotone, even fashion but dialogue is rarely delivered thus. Each person has a unique way they speak but this will never come across fully in prose. There could also be added meaning in where pauses are placed and which words are uttered slower than others. A competent recitation of an audiobook could go a long way but there is no replacement for an actor playing each character giving it full attention and a complete portrayal.

Pronunciation, Accent

We might think we know what an Austrian accent sounds like but in our heads we will likely cobble together something that isn't quite authentic. A trained actor will make the experience much richer. Often there are words we don't know how to pronounce or the character doesn't know how to pronounce. It helps overall comprehension as well as understanding a character better to hear him or her to say the words, even if they don't know how to say them properly.

Non-Word Noises

There are myriad sounds people make in conversations that can't be precisely put in text. It might take many sentences to describe a simple sound made between "Hello" and "how are you?" that a performer can easily and simply communicate. Grunts, pants, sighs, snorts, laughs and clucking are a few of the sounds that can only be minimally described in prose that can come across vividly in an acting performance.

Subtle Simultaneous Actions

We don't often simply speak to others without doing anything else. We don't hold our faces still as we deliver dialogue and we don't keep our bodies perfectly motionless either. Characters will perform countless subtle actions they think will aid in whatever their dialogue is trying to do for them. Characters often do these little things that don't warrant mention in a prose work but nonetheless add to the conception of character and story.

All these barely perceptible additions to words characters speak would take space to portray on the page but are effortlessly added in performance. In the name of saying the most in the least, it behooves you to consider whether what you're writing is being written in the right place.