Writing believable dialogue in fiction is a long way from dialogue in 'real life' which is peppered with a chaos of ums and ahs, you knows, basicallys and many more superfluous words and fillers. If used in a novel or short story these fillers will only serve to slow the flow and frustrate the reader. Likewise in reality we eat, watch television, cook meals, bathe, spend and waste time on the Internet, visit family, friends and sick relatives, shop and clean. But if included in our fiction (unless absolutely crucial to the storyline) it would drive the reader as far away from your novels as possible.
In fiction every piece of dialogue in a story is a means to a narrative end. In real life, conversations can be one sided, boring, animated or something used to avoid silence. Developing an ear for dialogue is good but writing it you must keep in mind the tone of the novel and the character speaking.
And on the subject of character dialogue many readers have told me that they dislike excessive swearing in novels, a view I personally share. In particular the visually impaired readers who listen to books on audio tell me this is very off putting. I also dislike excessive swearing in films and feel that sometimes a good film or television programme is ruined by it and it is completely unnecessary in fact a lazy way to convey an emotion. Yes, there is swearing in my crime novels but not excessive by any means and only in keeping with the character and the situation.
To return to dialogue and life, Alfred Hitchcock said that a good story was 'life, with the dull parts taken out' a viewpoint I am inclined to agree with and dialogue should follow the same pattern: it's human conversation without the ums and ahs.