Fapping to porn on a regular basis creates an area in your brain that is centered on getting easy, very big amounts of dopamine from sexual stimulus. Human neurons like dopamine, it's like sugar to a housefly. The problem is that over time, this area keeps growing and growing until ultimately, fapping to porn is pretty much the only thing "useful" you do in your leisure time. Instead of learning foreign languages or playing chess, you spend an hour looking for the perfect video to fap to. After fapping, when you feel released and try to learn Japanese or play chess, you find that your interest for these matters is considerably low, lower than the idealized representation of your interest, and you find your attention constantly fidgeting to more immediate sources of pleasure, mirroring porn: youtube videos, political controversies on the internet, reddit posts, this forum, etc.
There's nothing wrong with porn and fapping per se: it all depends on your life goal. Globally speaking, porn and fapping will maintain you where you are now: it is a source of life stagnation. That makes porn relatively harmless for those whose life is already okay, but harmful for those whose life sucks or have high ambition for something.
And before a fapper contradicts me with "I fap to hardcore porn daily and have self-learned 12 computing languages": assuming it's true, good for you. But all things being equal, fapping does decrease, in my experience and the experience of many others, the ability to focus on boring/hard material.
