No discernible reason?
Jack and Rose's story is a basic star-crossed lovers story, sort of echoing Romeo and Juliet. Like any of these rich/poor stories, you know that there are going to be hurdles for them to overcome and that if their love is to be accepted then they'll have to win over their friends and family. So all that seems like a pretty basic romantic storyline EXCEPT we're on the Titanic. So, from the first frame of the movie, we know how things are going to end. This brings a sense of inevitable tragedy (which in another story, at its best, would be a sense of foreboding) to their story, but also ramps the tension up. Not only are we focusing on their blossoming romance, but we're constantly on the look out for the point at which the ship is going to sink and their romance cauterised. And then, when the ship does sink, we have a massive third act that, ignoring the fact that it draws in an entirely new box office demographic, reenforces out commitment to the characters. We want them to survive! And we know that, in the case of the Titanic, there were hundreds of survivors and we already know that Rose has survived from the frame narrative. Whilst we know that her romance with Jack ends, we don't know how and we don't know what the significance of the ring is. We have invested an awful lot in the story of this woman who is standing above the wreckage of the sunken ship- and that final event is every bit as heart wrenching and evocative as it ought to be. This is a Shakespearean story where the audience knows that, at some point, an iceberg is going to come smashing into the plot and if that's not at least a little bit interesting, then I don't know what is.