Think of ruts dug into the mud. It is much easier to drive your car with the ruts than trying to drive through, over or against them.
Cache memory in a computer is another good analogy. The computer stores commonly used actions where it can access and process them faster. The brain does the same thing.
Setting up these pathways is actually quite simple. With daily repetition, you can institute a new habit in as little as 10 days. That is, of course, if the new habit is not competing against an old one. Unfortunately, once a pathway is set it is extremely difficult to get rid of, which is why breaking a bad habit is so hard.