Dialogue: The Good Parts Version

Everyone has heard that it's important for dialogue to sound natural. Under most circumstances, you don't want your characters to sound as though they're reading from a book. They should also sound like individuals, not simply an extension of the author's prose voice.

That said, dialogue is a refined version of how people speak in real life. First of all, we use, like, a lot of filler words. You know what I mean, right? In real life, we barely notice this filler, since we are focused on absorbing the content in real time. Reading, however, is slower, and we will not only notice filler words that appear at the same rate they do in real life, but be annoyed with them. As a corollary, excessive swearing in dialogue quickly becomes this-- meaningless 'white noise' words the reader has to skim through to get the meaning of the speech itself. 
Second, we say a lot of inane stuff. Psychologists refer to our social natter as 'keeping the channel open'-- using chitchat about the weather or other light topics to show our interest in the other person and keep the atmosphere generally friendly. While this is pleasant in real life, it's boring to read and slows the pace of your story to a crawl. Ideally, any small talk which seems necessary to the setting can be either summarised or used as a vehicle for some other conflict-- perhaps an assassin is trying to make casual banter while keeping an eye out for their target, or two people are engaged in flirting or passive-aggressive jabs for power. 
Dialogue should be the 'good parts' of conversation, moving information and creating conflict where it's essential to the story.