Because his male member (penis) wasn't giving him any rest (constantly horny) he decided to not give it any rest in return (frequent masturbation)
It's describing something we would normally consider embarrassing in a very dry way. Dry implies "boring" or difficult to read because it engages very little in the "feeling" department. It's what makes it funny to some of us reading it.
You're lucky the no-fap community isn't here else you'd be receiving essays with citations from a mf that failed college.