There is nothing that makes one happier then the promise of something they want or something they know will be worth it. The time it takes from the point you are promised this to the point which you realize it will never happen is how to gauge how much it will hurt/scar/or destroy your trust in that person. False hope can bring even the happiest or most chilled person crashing back down to earth.
Guys and girls if you promise someone something follow it though, you wouldn't want to be on the receiving end of false hope so why put others in that position