Living on assumption and promises is more like spending money you haven't received. People can make promises but how sure are you that the promise will be fulfilled.
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.
Why should any individual be living on promises and assumptions when we know everyone can disappoint you at any point. Sometimes the disappointment can be intentional or due to situations not under their control. For example someone can promise you a job but later on, the job is no longer available because someone has given the job to another person and it might be his manager or a superior in the office that gave the job out.
We have to learn to only spend money we have at hand, it's okay to plan for money you're expecting but that should just be only plans and not spending it when it hasn't arrived. When you plan it helps you to spend wisely but never have an assurance on something that isn't in your possession yet