To me, Julie is a girl who lives for now, and doesn't stop to think about possible consequences. The speaker has fallen for her, and she basically tells him that she doesn't want to be in a relationship because nothing lasts forever, and you can't be certain that you'll have a tomorrow. So then the guy starts to believe her, and thinks 'what good is tomorrow without a guarantee?' and he basically starts to believe all the stuff she was saying. And then he says to her 'let me be your guarantee' meaning that he'll be her guarantee of a tomorrow. Ok, that doesn't really make a lot of sense :) and i kind of repeated myself. But yeah, hopefully that isn't too difficult to understand.
To me, Julie is a girl who lives for now, and doesn't stop to think about possible consequences. The speaker has fallen for her, and she basically tells him that she doesn't want to be in a relationship because nothing lasts forever, and you can't be certain that you'll have a tomorrow. So then the guy starts to believe her, and thinks 'what good is tomorrow without a guarantee?' and he basically starts to believe all the stuff she was saying. And then he says to her 'let me be your guarantee' meaning that he'll be her guarantee of a tomorrow. Ok, that doesn't really make a lot of sense :) and i kind of repeated myself. But yeah, hopefully that isn't too difficult to understand.