If you commit to doing something, it is vital that you follow through. A gentleman doesn’t break his bonds. If you give your word that you will do something, you must do it.
If it’s in your power, and you’ve promised to do it, you don’t have a choice.
The flip-side of this, of course, is that you should be cautious in what you promise. A gentleman’s word can be trusted because it is rarely given. Your usual answer should always be “maybe”, “I can’t guarantee”, or “I’ll see what I can do”.
"I promise" or "I will" should rarely leave your lips.