I think this one is pretty clear. He had a great relationship and they promised they would stay together. You only stay with someone until you don't. He still misses her and wishes they had kept their promise so he imagines what it would be like to still be with her.
I think this one is pretty clear. He had a great relationship and they promised they would stay together. You only stay with someone until you don't. He still misses her and wishes they had kept their promise so he imagines what it would be like to still be with her.