I will give you #5000 on Monday, how sure are you that the money will come in on Monday? Promises and assumption has caused alot of people there job, imagine leaving your recent job because a friend promise to get you a better job. This may even cause someone his/her life if one base most of his/her lifestyle on promises and assumption.
If you easily believe such promises and you act ahead to celebrate then that's your problem. When something is not concrete and you're only holding on to their words, don't be hopeful and rely too much because it's not certain yet.
This is the problem of some people who plan and spend ahead even what they count on is still uncertain. It can lead to disappointment and problems if you already act and expect that it will certainly happen. Remember the saying
"don't count your chickens before they hatch", it means don't celebrate if you don't see the result yet. Nowadays promises can easily break of people who are not true to their words and though it's a human nature to trust easily, we should still be wise everytime.