The science behind memorization, learning, and similar functions in the brain are familiar to most people. School has taught most of us about neurons and how messages use them to travel throughout the brain. Most of us are also aware that the more a neuron is used (repetition of a behavior or thought), the stronger the connection becomes. This is when memorization, second-nature, and habits become real and lasting, as long as those neurons are being used often.
Now think of how addictions form and the similarities between creating a habit and becoming addicted to something. An addiction changes the brain in that the connections become stronger, the behavior becomes a habit, something the person becomes used to on a regular basis. Porn addiction is dangerous for just that reason. Viewing graphic sexual images and videos repeatedly creates pathways in the brain that only strengthen and change the brain for the worse.