Question: Someone in my writing group commented that my protagonist isn't very likeable. He's really not a very nice guy, does he have to be likeable? Can't he just be who he is?
Answer: Usually the reader identifies with the protagonist. He is "for" the main character, who is usually a "good guy." In some cases he may be an anti- hero. Someone who finds himself acting unexpectedly in a heroic manner, and we can all identify with an underdog, most of us having been one at one time or another.
There are exceptions, of course, but main characters who lie, cheat, steal, murder or otherwise do bad things are difficult for readers to identify with. Readers aspire, yes, nearly all of them, to be admirable. So if the main character isn't nice-- or at least motivated well enough so the reader can say, "I might have acted like that-- then they will have difficulty identifying with that main character.
Even those who love literary stories read to experience vicariously lives, other wheres, and other times. It was no fool who said, "We are the sum of our experiences, not the sum of our possessions."
When we create a story, we also create an experience for our reader. Not all such experiences have to be pleasant, but when they are unpleasant, we must give the reader plenty of reason to stick around. Scarlet O'Hara wasn't "nice" but we all understood why she acted as she did.
Understanding why the characters act as they do is often the key.