There are several things that could be stated about how the progression of the movie added to the enjoyment for the audience. So, I will have to be selective.
First, the development of love between Rose and Jack certainly added to the enjoyment of the movie. What made this development so delightful is that two worlds were colliding. On the one hand, there was the rich world of Rose with all the social pretensions. On the other hand, there was the real world of poor Jack, who really had nothing more than the clothes on this back.
Second, the love triangle that existed also added to the enjoyment of the movie. Rose was nearly engage to Calvert (Cal). He is a character that the audience loved to hate. His showdowns with Jack were memorable.