Man has ever a silent listener at his side - his subconscious mind.
It is his faithful servant but one must be careful to give it the right orders.
Every thought, every word is impressed upon it and carried out in amazing detail.
The reason it is necessary to make an affirmation is because repetition impresses the subconscious.
You cannot control your thoughts at first, but you can control your words.
"Be transformed by the renewing of your mind." (Romans 12:2)