Apart from controlling your breathing, regulating your temperature and ensuring your heart beats - your subconscious brain's primary function is to keep you safe. It relies on past experiences and beliefs to act upon what your conscious brain tells it.
Think of your mind as if it were a flower garden. Your logical brain or conscious mind is the gardener sowing seeds (thoughts) in your subconscious. If those seeds are given time, attention and nourishment they will flourish and grow into flowers. This is the case with both negative and positive thoughts. What your conscious mind thinks, your subconscious mind makes it happen, like a ship's captain navigating a ship.
During hypnosis we are giving suggestions to the subconscious mind to sow those seeds to grow the type of flowers that we want in our gardens. We are the landscapers of our own destinies.