Well, the enjoyment of this film is greatly enhanced by the atmosphere and mood that is created by the action that we see depicted as we meet the various passengers on the supposedly unsinkable ship and understand the various dreams, hopes and situations that they face. In particular, to answer this question you need to focus on the way in which the mood of the film gradually becomes far more disturbing as we understand more about the situation that Rose finds herself in. She is being forced to marry a man she doesn't love in order to save the fortunes of her family, and faces great pressue from her mother and therefore has to conduct herself appropriately even when she would rather behave differently.
As Rose is faced with the limited opportunities that she faces, this leads to a real shift in mood and atmosphere as the few moments she has enjoyed where she has been liberated are sharply curtailed and repressed. Of course, what greatly adds to this effect is the way in which the crash into the iceberg represents a change of mood and atmosphere as, on the one hand, it gives Rose hope of changing her life away from its doomed course just as the ship on which she is on is unable to do avert the disaster that awaits it. This offers us hope that not all endings involve a iceberg and a watery grave.