A Promise is a frequent theme in anime, manga, and real life.
A promise is like a vow between two people or more. A promise can be on anything, but a promise is a serious vow to keep. In Shounen franchises, a promise is serious when characters vow to stay alive, come home, or beat their enemy.
Mashiro and Miho promise each other they marry one another if they can fulfil their dreams.
Son Goku promises ChiChi he'll marry her when they grow up. Years later ChiChi shows up reminding him of his promise, which he had forgotten and tells her he thought marriage was some sort of food. He fulfills his promise anyway and they get married.
Even though Natsu Dragneel forgot his promise to the city of no sound, he completes his promise with the help of his guild to defeat Daphne and her magic.
Keitaro promises a girl he'll get into Toudai and he'll marry her when he does.
Completed Promises: Naruto promise a young boy to defeat Zabuza, Haku, and other ninjas. Current Promises: Naruto promise to Sakura that he will bring back Sasuke. (The series has not ended yet)