A 12 year old boy wants to wear diapers because they are comfortable to wear and disposable diapers feel even better when they are wet than they are dry.
Wet diapers are also good for masturbating in and it makes for easy cleanup.
The 12 year old boy probably likes to rub his penis in the wet diaper and get off in them which they are great for.
I'm an adult and have loved masturbating in a very wet and squishy diaper since I was 12 years old and Loved wearing diapers even before 12 years old.
I wear diapers all the time now and no longer use the toilet and wouldn't have it any other way.
Diapers are the best most comfortable underwear to wear and you can wet and poop in them as well.