REASONS WHY explination:
The reason why is beacuse Anime boys are perverts.
even when they son't want to be one they are.So anyway they get noebleeds for two main reasons.
Reason 1:the first reason is if they see a girl naked.
Reason 2:They see a realy pretty girl and think perverted thoughts.
Those are the two main reasons but theres many più such as finding a girl in ur bed.Or even seeing a pretty girl.Also it could be like in Naruto when they read perverted books.perverts could be just if they see a big chested girl.And it And te know they always get beatup after being perverts (which as a girl i would beat them up too!)Well i hope this answerd that domanda which i know più than half of te already know.
THANX!!