You may not know it, but the porn industry has figured out how to hijack your brain's normal circuitry to make you addicted to their product. And they are quite successful! Just as McDonald's uses our brain's cravings for calorie-dense food (that came from eons of living through food shortages), the porn industry uses our brain's natural mechanisms for bonding and procreation to make us crave highly sexualized, ever changing pornographic material. 

Here's how it works (I am paraphrasing from Sam Black, author of  The Porn Circuit): 

• Your brain has a natural chemical (dopamine) that is released when you do something pleasurable. This is to make sure that you continue to do things that are good for you, like eating or having sex (good for you in the survival-of-the-species sense of the word). This dopamine "reward circuit" is responsible for many kinds of addictions, from gambling to sex to alcoholism. According to Black, when dopamine is released into a certain part of the brain responsible for "emotion and learning, [it gives] the viewer a sense of sharp focus and a sense of craving" as well as pleasure. This causes the brain to create a pleasant memory of the experience, so that if circumstances arise that predict that the experience is near, the brain will release the dopamine again. It creates a learning loop where the brain tells the person (according to Black) “Remember where you got your fix last time. Go there to get it.”
• Another brain chemical that we normally think of as being used for attention is also at play. This chemical is norepinephrine. This chemical helps us to stay focused and pay attention to what's important. It is also released during sexual response, again reinforcing the idea that sex is important. And for the survival of the species it certainly is. But again the normal sexual response and the brain's response to that is hijacked by pornography, which the brain cannot differentiate from healthy sex. 
• As if that was not enough, the brain also releases two more powerful chemicals, oxytocin and vasopressin, during the sexual response. Oxytocin has been called "the bonding hormone" and helps us feel close to, and protective of, the person we are engaged with. It is the hormone that is released during nursing. However in combination with vasopressin, according to Black, they "help to lay down the long-term memories for the cells." In effect they tie the sexual pleasure memory to the person or thing that caused the sexual response.  Again in terms of mating and reproduction this is a good thing. It makes the couple stay together to raise offspring. However, in terms of pornography, it binds the person to the porn that they are using in very powerful ways. 
• We have all heard of endorphins, the chemicals that our body produces in response to pain. They are the bodies natural opiates and much more powerful than anything man-made thus far. This is what produces a "runner's high" or causes the injured soldier on the battlefield to not feel the pain of his wounds so that he can run to safety. This powerful chemical is also linked to sexual response and is released into the brain after orgasm. This can create a strong sense of well-being and pleasure that makes one want to repeat the experience, much like how the "high" of opium creates addicts. In love we become "addicted" to each other, a positive experience that can fuel a relationship. However when porn is the stimulus for this response, it creates a powerful craving for more porn!
• And finally, orgasm also changes levels of  serotonin. This is the chemical that is altered by antidepressants and is linked to mood (among other things). After orgasm, changing levels of serotonin create a feeling of calm and relaxation that is quite pleasurable. Again this causes us to want to repeat the experience. If the experience is having sex with a partner in the context of a healthy emotional bond, that can be very positive. However, if the experience is after viewing pornography, again the system gets hijacked to make sure you want more porn-- and soon!

So you can see how our brains are wired for partner sex. Each sexual experience, which based on mother nature was supposed to happen with another person, was designed to bond us emotionally to that person. To create powerful feelings of pleasure, excitement, affection and desire/craving. But that system gets hijacked by porn and turned into a powerful reinforcement system which leads the person back to porn for more and more "fixes". 

For more information brain circuits involved in pornography, download The Porn Circuit for free.

Below are other findings from brain researchers that illustrate how the brain responds to pornography: 

Cambridge Neuropsychiatrist Valerie Voon, in the UK documentary Porn on the Brain, demonstrates that the brains of chronic porn users closely resembles the brains of alcoholics. She explains that her research has shown that a particular area of the brain, the ventral striatum,  “lights up” when a porn addict sees porn. This is the same area of the brain that responds when an alcoholic sees a  drink. So the same areas of the brain that get hijacked in alcoholism, leading to addiction, are being hijacked in people who repeatedly use pornography. 

And in an interesting book by Dr. William Struthers  (Wired for Intimacy) brain research is quoted as demonstrating that viewing pornography and masturbating changes the singular cortex. This area of the brain is responsible for helping us have willpower and make hard moral and ethical choices.  The singular cortex is actually weakened in people who view pornography habitually. The sad fact is that pornography habits make it harder for us to avoid going back to pornography! It becomes a self-perpetuating cycle. 

Gary Wilson, in his TEDx talk, explained why people who use porn frequently eventually have to find more and more extreme images/experiences to get the same result. As noted earlier, porn causes a release of dopamine in the brain. After using the same kind of porn over and over again the brain actually begins to wear out! It stops the production of dopamine to that stimulus (or anything that is too similar), which leaves the person craving their dopamine "fix" but unable to get it. The only way to get the high back is to find something that is more extreme than the original porn. Think of this like an alcoholic developing tolerance to drinking-- at first two drinks gets them tipsy, but in a year or two it takes 4, then 6, and so on. So the tolerance to the original level of stimulation drives them to more and more extreme types of porn just to get the same feeling of pleasure. 

For more information on the consequences of this particular aspect of porn addiction see the article "Why Does Porn Seem Hotter Than My Partner?" (http://goodmenproject.com/featured-content/why-does-porn-seem-hotter-than-my-partner/) One of the most important things I have learned as a therapist in this situation is that chronic pornography usage can actually cause impotency in real-life sex! The brain gets so used to responding to porn that it gets confused when we have a real-life partner. Men can actually experience impotency or other erectile problems when they try to have sex with a real person, because they have conditioned their genitals (and brain) to respond only to a virtual person!

For more information on how to undo the porn habit and the hold it has on your brain (and your body), you may want to look at The Porn Circuit: Understanding Your Brain and Break Porn Habits in 90 Days.

The take home point is that Mother Nature never anticipated the internet. So our poor brains are not doing a very good job of keeping up. Circuits and chemicals that were designed to help make us get together to procreate and bond to partners are going haywire when exposed to types of stimulate they never anticipated. If you want your relationships to last you may need to learn more about the detrimental aspect of pornography and how to put those same circuits and chemicals to use in favor of your relationships rather than against them.