When speaking of the emotional dagger to the guts that was Jack’s death in Titanic, I do believe that Billy Zane had the right idea when he kept his explanation simple and to the point. Jack had to die for the story to make sense, it’s about as short as it can get while still offering an explanation of some sort. If Jack had survived the narration and the story itself would have been far different and carried much less of an impact for several reasons. It might be unbearably tragic and has in fact generated theories ever since as to whether Jack could have joined Rose on the door, but the pure and simple fact is that Jack had to go.
In love stories the happy ending is what people want to see, but sometimes the happy ending is only a precursor to what might become a long and drawn out existence that would mean less than a singular existence spent remembering someone that had been lost. Rose was a very strong-willed woman after meeting Jack and remained so for the rest of her life. She went out and lived in the manner that she wanted when she reached the United States, and had Jack survived there is a good chance that she would have settled down with him to lead a life that wasn’t bad, but wasn’t as fulfilling. Had this happened the entire second half of the movie would have been wasted, and one of the best parts would have been just about meaningless.
There was no record of Jack ever boarding the Titanic as he and his buddy won their tickets from two others in a poker game, remember? They weren’t on the manifest under their own names, and wouldn’t have even existed according to history. The only place that he truly existed was with Rose, and as she mentions as an older woman he exists only in her memories. Had he survived the effect of finding the Titanic wouldn’t have been that great, and the drawing found would have been more of a remembrance between the two of them, not so much a reason to pick up the phone and call the people on the boat. In fact there’s no way to tell if Jack and Rose would have even stayed together or if he would have been alive at that point since she was an old woman when the picture was found.
There’s also the idea of the tragedy of the hero’s death leading the story in the desired direction. Jack stayed with Rose until the end, when his heart no longer beat and she finally had to let him go. He sacrificed everything for her in the hope that she would continue to move forward and live a life that was her own, not what anyone else wanted for her. His sacrifice meant that she not only went out into the world and experienced it to the fullest, but that she kept her promise and in doing so, upon her passing, was rewarded for her diligence and faith. Keep in mind that the final scene, when she reunites with Jack and the rest of the crew, that this is one of the most fitting images of any movie and the grandest among many endings.