“The answer is very simple because it says on page 147 [of the script] that Jack dies. Very simple. Obviously, it was an artistic choice,” Cameron, 64, said at the time. “The thing was just big enough to hold her, and not big enough to hold him …  I think it’s all kind of silly, really, that we’re having this discussion 20 years later.”

Cameron then noted that Jack “had to die” because Titanic is “about death and separation.” He also added that “had he lived, the ending of the film would have been meaningless.”

Source: US Magazine