Contrary to popular belief, Jack was entirely made up. Joseph Dawson was a real passenger on the real ship - and he had no connection to Jack Dawson, nor was Jack based off of him. James Cameron (the director of Titanic) had no idea Joseph Dawson existed whilst he was making the movie. Please don't take this the wrong way though, haha. I'm not trying to be a smart alec, just trying to clear up a common misbelief.
Who wouldn't fall for a Jack Dawson? If I were Rose, I wouldn't mind as well if he's a third class passenger. The "I'm the King of the World scene" is a classic. Oh, I wish I could meet my Jack one day.