James Joyce's Ulysses, for some, is a very hard text to understand. It is understandable that you are struggling to find the "greatness" in it. (If one fails to understand a text then they are simply unable to grasp the importance of the work and see why others have named it as "great."
Joyce's text is one filled with interior monologues which allow the reader to inter into the stream of consciousness of the different characters depicted throughout. readers without a distinct understanding of the purpose of the interior monologue tend to become buried in the, sometimes, dialogue.
The text has been determined great based upon its ability to transcend the human psyche and examine the human condition (a person's fears about death and isolation, the search for self-identity, and their curiosities about life in general).
Do not be discouraged at the fact that you cannot find the greatness in the novel. Unfortunately, no one can explain to you the greatness of the novel. It is simply impossible. Greatness needs to be defines by the individual reader. Perhaps the best way to describe the text itself is through a quote provided by Edmund Wilson. According to Wilson, with Ulysses, Joyce has
written some of the most unreadable chapters in the whole history of fiction.
Therefore, do not be discouraged. Many have fought with Joyce's Ulysses and failed at understanding it.