The neurochemical dopamine is the central player in all addictions. Most every article on addiction, reward, motivation, or learning mentions it. For example a Google scholar search for dopamine + addiction returns 200,000 items. Dopamine dysregulation is at the heart of porn addiction, cravings and withdrawal symptoms. Restoring normal dopamine function and sensitivity is a key to porn recovery.
This section contains both lay articles for the general public, and research articles. If you are not an expert in addiction, I suggest starting with the lay articles. See the letter “L” at the start of those articles.