The key word here is "boast." Waltke points out that the word can mean "excessive exaggeration," "spontaneous" or volunteered bragging. In the vernacular we might say one who speaks as a loud mouth about what he is going to do tomorrow. The Rabbis point out:
Lawson adds, "In God we live and move and have our being; but we too often forget this important truth, and speak, act, and think, as if we live and move and had our being in ourselves. We boast of what we will do, or of what we shall enjoy at the distance of days, and months, and years. This presumption is forbidden in this and many places of Scripture."
Unger points out that "bring forth" is an expression of child bearing. We do not know what will be birthed for us tomorrow! On "boast" he says, "Do not flatter yourself about tomorrow or brag about it." You do not know God's plans for your life the next day!