i think he knew that he was going to die if a boat wouldn't come back soon. but he was very weak and wanted rose to give him this promise. i think you're right, hi didn't say i love you too because in this case it's like a goodbye. jack was confident with his life, rose absolutely not and she wanted to start a new life with him. but he knew that only one of them could survive(if the boat would've come a bit later rose woul've died too) and he wanted rose to do that so that she could see how great live can be (unfortunately without him). and so he died for her. that's much more than these three words.
Remember when the boat came and rose recognized that jack was dead - she wanted to stay and die. but than she remembered the promise i think