We used to be able to eat something that would give us a small reward, like a grape. But now we have man-made substances that have about two times the amount of dopamine than natural ones. So instead of having sex with one person one time and getting that dopamine release, we can now watch hours of porn and get a huge dopamine release. Your brain prioritizes these big rewards over the small rewards. That's why your brain craves porn more than that sexual connection with your spouse.