The meaning of the exhortation and warning is the same as the meaning of Christ's parable about the rich man whose land produced plentifully (Lk 12:16-21).
"Come now, you who say," and so on. People are presented who reason as if their life and their affairs depended solely on themselves.
They act as if there were no higher will above them, which can turn all their plans and projects into nothing in a single moment, entirely without their knowing it.
The Apostle points out the absurdity and irrationality of such an attitude toward the affairs of life. Knowledge even of the nearest future is hidden from man, namely, "what will happen tomorrow."
Compare Lk 12:20: "Fool, this night your soul will be required of you." Moreover, human life in general is brief.
"He does not destroy free choice, but shows that not everything depends on man himself, and that grace from above is also needed. One may travel, trade, and do everything necessary for life. Yet one must not ascribe this to one's own labors, but to God's love for mankind" (Theophylact).
"You do not know what will happen tomorrow": that is, whether you will still be alive (Prov 27:1). Yet you draw up various plans for acquisitions, and for whole years (v. 13).
"He shows the vanity of our life and puts us to shame because we spend our whole life in vanity. All our labor is exhausted on temporary evil.
"David says the same: Man walks about as an image, yet he is troubled in vain (Ps 38:7). That is, he busies himself over what in itself has no true being, but appears only as in a phantom" (Theophylact).
Our life in general is a vapor that appears for a little time and then vanishes as quickly as it appeared (v. 14).
The appearance and disappearance of vapor do not depend on the vapor itself. So also are the beginning and end of human life, which in general is brief.
Its beginning, continuation, and end are in God's will. Therefore a correct, rational, and religious view of one's undertakings and intentions must necessarily rest on this thought.
If the Lord wills, and if we live, then we shall do this or that (v. 15). Everything is in God's will: man's life and the fulfillment of his various intentions.
If it pleases Him to allow us to do this or that, or even to remain alive, He will allow it. If it does not please Him, we cannot do it, and at any moment we may even be deprived of life.