When you speak positive things and have faith in God, things will work in your favour according to your confession and heart desire. This is because, there is power in your tongue. But if someone speaks negative things about him/herself, things may not work in his/her favour because negative confession attracts negative conditions to someone. Let us think about the word of God in Proverbs 18:21.
Death and life are in the power of the tongue: and they that love it shall eat the fruit thereof.
-Proverbs 18:21