Your brain needs endorphins and encephalins, (naturally occurring opiates produced in the brain). These opiates make you feel good.
There are manufactured substances that mimic these naturally occurring chemicals (drugs, alcohol etc.) but there are many healthy, natural ways a person can get these brain-chemicals.
When a person gets these ‘feel-good’ chemicals, they want more—these chemicals can become as addictive as alcohol or other drugs.
Sexual feelings and desires are a natural part of each person. The pathway of sexual arousal and release is one of the most powerful pathways with little need of reinforcement. Because it is so powerful, it also needs the most protection so that it can occur in the situation that is the healthiest for those involved.
When a person views pornography (sexually explicit material for the purpose of arousal) all those feel-good brain chemicals are running all over the brain. When the individual becomes aroused, and feels a need to experience sexual release (usually by masturbating or having sex.)
When pornography is combined with sexual release, as in masturbation, and the naturally occurring drugs are set free all over the brain, this is a high reward for the brain. Because it is not a chemical imitation, it’s even more addictive.
(Source: Brain Pathways, Douglas Weiss, Ph. D.)