I mean, Rose was about to die - she was on that wooden door in the freezing cold and had no reason to live anymore... She just wanted to make sure that Jack knew she loved him, and that he loved her, too. If he had done nothing than saying yes, she would have thought: "Uhm, okay, he knows I love him, and he loves me, too, so I can go now..." Jack knew what she thought then, and he instantly tried to make her "go on" ("Promise me, now, Rose." and so on).