Promises are a double edged sword. Once you have it there is a chance that it can slip through your lips and spill forth into the world where it does not only embarrass you but the person of the secret contains. Though it is meant to be secret he knew that some things he would have to keep away from all he knew. Including his mother.
Promises held you to a world that is not your own. This secret involved himself and his best friend, Draco Malfoy. Though he made it to the Malfoy heir when they were just children he knew that it was one that he would keep for the rest of his life.
Protect his friend's status and sanity before and after the Dark Lord comes into power.
His family may be considered neutral but he knew he would have to choose a side eventually.
Remember your promise.
That is what Draco told him when he was tasked with stopping the Golden Boy from destroying their leader. With a calmness unlike any other he walked towards Draco and said "What needs to be done?"
The impossible.
They quickly headed toward the Room of Requirement and waited amongst the knickknacks of Hogwarts for the wondrous Golden Trio.
Nervousness wasn't supposed to course through his veins but it was. He didn't want to follow the Dark Lord's rule during this time and he paved in the hope that Harry Potter would win and the Wizarding world would once again will be at peace. Though he could only speak it in silence he wanted this war to be over once and for all.
But promises had many lines and many chains that had many followers. He looked toward Draco and sighed.
Promises like unbreakable vows were completely unnecessary on certain occasions.