This is a simplification, but your brain connects sets of neurons when you perform two actions one right after the other. If set A fires to brush your teeth, then set B fires to get you in the shower, A and B start to form a physical connection. The more you brush your teeth, then get in the shower, the more tied the A neurons become to the B neurons. If you do it enough, you’ll have to consciously prevent the B neurons from firing after the A ones. That’s automaticity—doing a thing so often that it becomes an automatic process for your brain.