Our speech will always indicate what is in our hearts. But when our hearts are filled with the goodness of God, praise for Him and love for others will pour forth. Sooner or later, the evil in the heart comes out through the mouth in curses and swearing. Jesus explained that what comes out of our mouths is that which fills our hearts. We cannot praise God while at the same time cursing our brothers. By making the analogy of both salt water and fresh water coming from the same spring (which is uncharacteristic of springs), he makes the point that it is uncharacteristic for a believer to have both praise and cursing come from his/her mouth. James makes it clear that the lives of Christians - the 'brothers' - should not be characterized by evil speech.
