Albert Barnes on James 4:16

But now ye rejoice in your boastings. That is, probably, in your boastings of what you can do; your reliance on your own skill and sagacity. You form your plans for the future as if with consummate wisdom, and are confident of success. You do not anticipate a failure; you do not see how plans so skilfully formed can fail. You form them as if you were certain that you would live; as if secure from the numberless casualties which may defeat your schemes.

All such rejoicing is evil. It is founded on a wrong view of yourselves and of what may occur. It shows a spirit forgetful of our dependence on God; forgetful of the uncertainty of life; forgetful of the many ways by which the best-laid plans may be defeated. We should never boast of any wisdom or skill in regard to the future. A day, an hour may defeat our best-concerted plans, and show us that we have not the slightest power to control coming events.