Denial

Everyone goes into some type of denial in their lifetime. A example is from F. Scott Fitzgerald, Great Gatsby. Daisy Buchanan is married to Tom Buchanan but she knows that Tom is having a affair but yet acts like nothing is going on. Could it be that Daisy is covering up from her own affair with Jay Gatsby. What causes a person to go into denial? Can denial be a reason for depression? Can we help someone who is denial truly?