Proverbs 27:1: "Do not boast about tomorrow, for you do not know what a day may bring."
Why is boasting inappropriate? Well, there's no question boasting goes hand-in-hand with the sin of a prideful heart. Also the intention of boasting can often be the approval of others. Ultimately, though, boasting tends to be a strategy to identify our self-worth without God and His presence in our lives.
Jeremiah 9:23-24 gives us license to boast in one thing... a relationship with the Lord.