There is a chemical hormone called Dopamine in our brain which judges how interesting is the activity you are performing when u do something for the first time dopamine secretion is at it’s peak and make u feel very excited and when u do some routine activity dopamine secretion is not quite much so u don’t feel anything special now the brain labels events as interesting based on how much dopamine is secreted when it occurs.
Watching porn videos is a very arousing and stimulating activity in itself and it is bound to flood your brain with dopamine and your brain will label it as a highly stimulating and interesting activity.
Now , whenever you will do something else or more productive thing than watching porn it is not possible to do something thrilling and ineteresting everyday for an average Joe so if dopamine levels are not high enough your brain will urge u to quit this activity and just masturbate watching porn and u will lose interest in any other activity and your passions will fade till u become a brainless sex obsessed person who will grow anxious if he/she didn’t watch porn for a day much like Opium addiction.