James points out several weaknesses that should cause us to be very careful regarding our speech when speaking about the future. The first is our ignorance of the future. James is not suggesting that we should not plan ahead. Instead he is rebuking assumptions that we can make about our involvement in the future, and also assuming that whatever the future will hold will turn out to be good. Connected to his rebuke is the issue of what are our priorities for the future. It is obvious that the plans devised by his readers were selfish.
The second detail that James mentions is the shortness and fragility of life, which he likens to temporary mist. Even the longest life is relatively short, especially in light of eternity. When it is over, what will we have? This is one of the issues raised in the Book of Ecclesiastes where the wise author considers the various achievements of life and asks what real benefits come from them. His conclusion in one place is that fearing God and keeping his commandments is what gives significance to life.
Why do people have this attitude to the future? It may be that they are trying to escape from the past, or it may be that their present is very unpleasant. Perhaps they want to use their talents in a way that would bring about security later on. Of course, in the process, they were forgetting to trust in God.
I would also suggest that there is another reason, a deeper one, although distorted because of our sins. It looks as if we as creatures were designed for looking ahead, and in a sense the gospel brings to believers the correct way of anticipating the future. We could say that those described here by James are people with a distorted or damaged vision of what can be. Instead of relying upon God, of adopting the biblical description of future glory, they instead imagine one that even if it does occur will not satisfy.
In contrast to them, we should be focussing on future experiences that God has promised to give to all who will depend upon him through the gospel. Somebody has said that if we live for this world we will eventually lose everything whereas if we live for God we will gain everything.