Do not boast about tomorrow, for you do not know what a day may bring forth (Proverbs 27:1 NIV).
There are things beyond man’s control. And they can act at any time. Those who boast about tomorrow either close their eyes to this fact or think that such things may happen to others but not to them. They also base their boasting on their present condition. They assume that their status today will continue to become better and they will thus live in the coming days. But these things may not happen according to their wishes.
Again, those who boast about tomorrow do not take into account God and His ways with men. They conveniently forget that the God who can build them up can also throw them down. He exalts one and puts down another. He can do so in a space of few hours; at times even much much less. Think of Joseph who was exalted from the prison to the palace in a flash. Think of King Belshazzar (Daniel 5) who dishonored God and was slain that very night; his kingdom given to Darius. Therefore tomorrow might dawn differently than expected for those who boast about tomorrow.
Then there is the aspect of knowledge to be considered. The simple truth is that “you do not know.” Yes, tomorrow is an unknown thing. No one knows whether tomorrow will come without them here. No boasting of knowledge acquired through years of study or scientific achievement or careful observation will be enough to know what tomorrow will bring. A new day might go silently by or it can throw a lot of unexpected and unpleasant surprises. It can crawl along and suddenly in a flash bring a lot of twists and turns. So better not boast about tomorrow.