Doing anything enough many times makes it a habit, an automated subconscious process. Habits are a natural part of the human experience.
In the beginning of our driving careers, when we are learning how to drive, we find driving to be fun and exciting at the same time as being slightly awkward and quite a bit scary.
The monkey is not an actual physical monkey, it is a metaphorical symbol for our innate animal nature.
“Got a monkey on my back, M-m-m-m-monkey on my back back back back” –Led Zeppelin – Nobody’s fault but mine