As Coue says, the child still exists in the most mature person. So the infantile mode of repeating the formula, "Every day in every way I'm getting better and better" tends to put you in touch with deep levels of the Unconscious where the child-mind still survives.
"Although these instructions no doubt fall somewhat short of our ideal of a thought entirely occupying the mind. But they are sufficient for a beginning. The soverein rule is to make no effort, and if this is observed you will intuitively fall into the right attitude. This process of the Unconscious adaptation may be hastened by a simple suggestion before beginning. Say to yourself, "I shall repeat the formula in such a manner as to secure its maximum effect.: This will bring about the required conditions much more effectively than any conscious exercise of thought."