Dopamine, a neurotransmitter that sends information between neurons, plays a large role in wiring the brain. It is released into the body when a person, or animal, encounters a stimulus that is novel or noteworthy or potentially rewarding. Dopamine performs many functions along many pathways in the brain, but in the context of pornography consumption, after a person is stimulated by pornography, dopamine is released along the mesolimbic pathway into the nucleus accumbens. The more dopamine that’s released here, the higher the likelihood the person will be drawn to the stimulus, or related stimuli, in the future.
A large amount of dopamine is released during natural human sexual activities and interactions; an orgasm releases significantly more. Pornography complicates matters for the human body: it is continuously available and ever-ready to serve as a sexual stimulus, with no bodily restrictions or limitations. As an endless source of sexual stimuli, no one human being can realistically compete against the accessibility of pornography. And so the process of consuming pornography literally conditions the brain to anticipate and desire a degree of stimulation that cannot be duplicated in real life, of specific imagery and behaviors that perhaps shouldn’t be duplicated in real life (reminder that 88.2% of all mainstream pornography regularly depicts physical violence, of which 94% is perpetrated against women).
Once the brain has fully learned to associate the stimulus with a response (such as a video with an orgasm), significantly less dopamine will be released. Consequently, the original stimulus will literally no longer be able to provide the same degree of anticipation or desire. Once a person becomes desensitized to the old stimulus, they need a more shocking stimulus in order to match the dopamine released in the prior stimulus. This need for something shocking or more extreme is called tolerance.
Put another way: A man enjoys the rush of masturbating to softcore pornography. After a while, he no longer gets the same rush he once did. He turns to hardcore pornography, and his rush returns. However, the same thing happens again. Before too long, he is watching gonzo pornography, and then a series of videos sexualizing children. Even as simplistic examples go, this is a fairly typical pattern for male consumers of pornography. This is exactly how the consumption of softcore pornography can lead to hardcore pornography, hardcore pornography to gonzo pornography, and so on. This is exactly how pornography rewires the brain.
(There are countless other factors which play a role in re-wiring the brain [opioids, the limbic system, and so on]. In time, they will be addressed and either added to this page, or I will reframe this question as a matter of dopamine’s role in rewiring the brain.)