I choose "Promise" as title of this fanlisting because all meaning of the song remind me a promise with a person that we can meet again in future, and that absolutely we can't lose.
The words of "A Little Pain" and the title too are sad, but the positive meaning give to us a reason to believe in the tomorrow and in the beloved person. Even if we're scared and sad, we always need to find a reason to wait for tomorrow and for happyness.
I want to explain positive and negative feelings in a single word, and "promise" is something made by a person but that only who really believe in it can realize, and if we make a promise, we have a goal to reach not only for ourself but for the person that made the promise with us.
It's always difficult get a decision as leave the beloved person in order to realize our desire or reach our goals, but it's necessary, and Nana and Ren's promise is the most representative example of this. Definitely, I think that this is the key-word of "A Little Pain".