Similar to “You are what you eat”, it is also true that whatever you say speaks about the person that you are. Whether it is normal speech between you and another person or the things you say privately to yourself, all will have an effect on your psyche consciously and sub-consciously. So similar that a person should stay healthy by carefully choosing the foods he eats, he should also pay close attention to what he thinks and says.
LK 6:43 "No good tree bears bad fruit, nor does a bad tree bear good fruit. 44 Each tree is recognized by its own fruit. People do not pick figs from thornbushes, or grapes from briers. 45 The good man brings good things out of the good stored up in his heart, and the evil man brings evil things out of the evil stored up in his heart. For out of the overflow of his heart his mouth speaks.