The admonition is to be alert and take nothing and no one for granted. Live life on tiptoe in anticipation, and in awareness of what is going on over the horizon.
Boasting vs Believing about Tomorrow
We are to make faith declarations and dream statements as freely as we breathe, but boasting is another matter. It presupposes that we are self-sufficient and need neither God nor other people to reach our aspirations and excludes gratitude from the success equation of our lives.
Thus, the writer of Proverbs declares (27:1): Do not boast about tomorrow, for you do not know what a day will bring forth." (New International Version)
Of course we do not know. There are variables beyond our oversight and capacity to predict. We do not control other people's choices or the heartbeat of nature. There are larger purposes than our own and we function best when we view our lives as part of a larger plan and submit ourselves to a greater will. When that happens, the dream that is planted within our hearts is a dream we must pursue and can pursue with divine guidance and power.
However, we do not know what valleys of sorrow or storms of trouble through which we must pass. We cannot boast about the time frame. All that we can determine are our choices and our beliefs and these we must control or they will be controlled by the circumstance.
You can believe in tomorrow without boasting about it.
You can predict your own success without presumption.
You can embrace a life of faith without assuming that there will be no battles to fight or hardships to bare.
You can control your decisions without controlling all of your circumstances. You can choose to be an overcomer without knowing what tomorrow will bring. You can live in absolute ignorance of the specifics of the future and remain positive if you know and trust the One who holds tomorrow.