Why does PMO defeat noPMO?

Because it feels better in the short term.

 

It makes more sense when you understand the mechanisms that drive the process.
You want to as you call it "PMO" when you have a need and want to use specific activity to satisfy it, need creates discomfort you may physicality notice or not. For example someone who wants sex may say - I am horny and I really want to have sex and I cant think of anything else. I try to do other things but thoughts about sex just come back and distract me. So body creates discomfort first and then it begins to offer reward to facilitate behaviors that will satisfy one or few of the needs it has.
Who is being rewarded? Well - part of us that experiences reality, its in essence biological computer program. So to control this program body has feedback mechanism in form of feelings.
In general if you feel good, you are probably doing the right thing, because your body is giving you "good" feedback. If you are doing something wrong - you generally do not feel good so it is time to change behavior and learn. So naturally we are confident doing things we know how to do - because we feed positive and confident - but we are extra alert, nervous and hesitant doing new things.

Putting this together when you have a need you feel good about doing things that satisfy it and feeling good suppresses some of your inhibitions but eventually you hit the other side when negative effects begin to give you "bad" feedback and you feel sad, anxious and bit depressed - in this sate you are able to question your actions and make plans for the future. Abuse of activities that alter mood create swings in how you feel and these swings lead typical "feel normal -> use -> enjoy -> feel good -> ops feel guild -> stop using -> feel ok -> feel normal - > use -> enjoy....

On thing I suggest it to make sure you do not connect "PMO" with your natural urge for sexual contact.
This desire for sex works by same principle, we want sex more and more when we have sex and may not want it again for some time. If you connected your desire for sex with "PMO" you may do that instead of just satisfying your need.

Just keep in mind that there is no consistency between the two, if you feel good - you have one set of traits and brain activity and if you feel sad you have another set of abilities.

Someone who is depressed may think about killing themselves even when surrounded by loving family and then take anti-depressant and truly enjoy time with them.
Its really a chemical and brain activity thing, we experience the world differently based on how we feel. And how we feel changes throughout the day and overtime

 

To add to Ronila's comments, it must be stated that the brain is quite ruthless when you're in the process of starting such a desired activity, because the same neural pathways are excited as when you watch porn for hours, so even if you still haven't chosen a video on a certain porn website you are already in too deep. Picture it this way; you decide to step on the ice which looks thinner because it still looks safe, but then you see the ice cracking beneath you after a few steps in, so there is a small chance you'll escape falling in even if you move slowly. Just don't let your ego lead you to peeking in on porn vids, cause you will fall back into porn.

 

Chaser effect, understand this