This question can be answered in a short and sweet way:
Because subliminal messages pass underneath the radar of our ‘conscious ‘thoughts and move into the realm of influencing our sub-conscious minds where the real decisions and life beliefs are housed.
That is the power of subliminal thought. You can use affirmations at the conscious level until you are blue in the face , however, if you don’t actually believe what you are saying at a deep subconscious level you cannot change what you are wanting to change.
Subliminal messages talk the subconsious talk. They speak in the language of the subconsious so that it understands what you want. Subconscious mind likes images, feelings, accurate wording, symbology, and it wants it quickly. That is how we ‘install’ things in the subconscious mind.
Instead of asking yourself “What am I thinking”? ask yourself “What am I feeling in association with what I am thinking”? The feeling is the part of the installing process which allows manifestation to occur. Subliminal messages allow this process to occur automatically .
