The movie is about their relationship, not their relationship as the embodiment of something greater. That's where the movie fails.
But it is Matthew. They are the link that brings you into the story, that lets the audience partake in what it was like on that ship all 4 days. Do you really think the movie would have been as interesting if it was
purely historical? Many of you are complaining even so that it was "boring"! Imagine how boring it would have been without that plot?
And I've already explained before what the relationship was about, I guess I'll do it again...
It pretty much signified how one person can change you for the rest of your life in just a few days. And the sacrifice at the end is what made the "union" between them able to last for 80 years. The fact he sacrifices his life for her, so she can go and do all the things with her life that she always wanted to, propels it form simply being a stupid love story. This isn't 'You've Got Mail' here. He fucking dies. He slowly freezes to death and dies, without a word of protest, so she can live. That is some serious fucking shit. I'm starting to think now that maybe one reason why you dislike the movie so much is because you know you
yourselves would not be capable of such a thing?
The story of Jack and Rose (especially Rose) is needed also for the entire motif throughout the film, and that is Time. The movie is constantly switching back and forth from present day to 1912. This experience of shifting between times keeps the film from simply being a period piece. We need that connection as well. We need Jack and Rose's connection, we need our connection to them, and we need the connection between our present day to that of 1912 to fully feel and understand how those people felt at that moment, at that night, at that presice point in history. And also to see how it relates back to us, now.