It is a childish fantasy to believe that a person can understand a situation fully enough to guarantee something to someone. This is particularly optimistic in the common form of promising a service to someone regardless of future circumstance. When someone makes the commitment of a promise, they claim to have identified with every possible outcome and are satisfied that they would react uniformly, no matter what happened. This is, quite frankly, ridiculous.
To give a common example, people often reassure one another with the phrase, “I’ll be fine – I promise.” Now, unless the speaker has the ability to see into the future, this promise is entirely unfounded. There are so many external factors affecting someone’s life that they have no control of. Technically, every person you meet could be a psychopathic killer with a deep thirst for blood, and you can’t possibly know that you won’t fall victim to this thirst immediately after exiting the door (there are, of course, many other less extreme example… but they’re just not as fun.)
If you’re not yet convinced by this example then let me give you another: “I won’t tell anyone – I promise” What you are saying with this promise is that if someone put a gun to your head and asked you for this precious information, you would politely decline on the grounds of promise prohibition. I don’t know about you, but for me this is a bit of a burden. If you’re the sort of person that lives and breathes promises then I urge you to change your perspective. It is unfair on your fellow man and also yourself to think that people should be bound to their word or else be some sort of horrible moral-less monster. This is just not how life works.