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, bathe, spend and waste time on the Internet, visit family, friends and sick relatives, shop and clean. But if included in our fiction it would drive the reader as far away from your novels as possible (unless, of course, it forms a critical part of the story).
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.
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.