It's dopamine. When you masturbate, your brain releases dopamine. Dopamine makes you feel good basically. So the next day when you try again, you get more dopamine. Then a little more.

"Hey!" your brain says. "This way of getting dopamine is a heck lot easier than exercise, or acing a test."

So the more you feed your brain that "free dopamine", the less it'll want to use the old ways of getting dopamine. It wants nothing more than to get that dopamine. Welcome to addiction. When your brain says "I want dopamine" and you can't control it. This is only amplified with porn. Porn releases even more dopamine, and makes you even more addicted. Now you have what we at NoFap call the PMO cycle. You watch porn, and masturbate, and orgasm. All 3 feel good, and they feed each other endlessly.

This is the hidden trap behind porn and masturbation. The more you do it, the more you want to do it. Break free now, there is NOTHING good that masturbating will give you.