The brain grows in size, limited by the physiological space of the skull, and so it grows connections between parts of itself. Even the shape of the brain as it curves in on itself and folds, allows it to make maximum use of its limited space.
When the brain is new it has connections everywhere and it has room for everything. As information comes into the brain, the brain drops some of what it has previously captured in order to make room for new information.
Since some information has more relevance to what our needs are the brain gradually starts to build up patterns of response to perceived behaviour as it prunes certain connections and collates others. Thus our brain’s preferentially of collated, stored information is the basis of our automaticity.