Rock and metal exists because people have something to say. Like, SAY! So what happens when an individual who has forged their career on catharsis gets happy?

It doesn’t matter if they are happy, sad or indifferent; artists almost never produce anything as edgy as their early work. The edginess comes from a combination of youth, angst and a nearly unlimited supply of pent up energy and emotion. As we age we can certainly be pissed off, but we just don’t have as much energy to keep the fire lit. Plus once artists like Metallica or Manson become millionaires, life gets much more comfortable. Musicians also tend to become more adept at expressing a wider variety of emotions in their songwriting as they get older, diluting some of that pure youthful anger.
Chad Bowar, About.com Heavy Metal

Virtually every metal band has been created be like minded rebellious youths who want to make brash music that goes against the grain. The concept of music being created by “angsty, troubled youths” is a figment of the media’s imagination. Sure there were/are problematic kids making heavy music, but I’m sure there have been a slew of boy bands with the same issue. You don’t have to be angry or depressed to write good metal, and any real metal fan can understand that. You think Saxon and Iron Maiden have always been old men?
Cody, All Metal Resource

Overall, metal is an angry kind of music. You don?t have to be an angry adolescent trying to figure out your place in this world to have the ability to write good metal. Adults can be as frustrated and disillusioned with the world as any teen. At this point, I?d much rather hear a band singing about their disillusionment with society (for example) than a kid angry at their parents or teachers. So do all good metal songwriters have a dark and troubled past? Do they all need to be messed up or angry about something to write heavy metal music? There some truth to this, but what can also feed the flame is a passion and a hunger for the music. Just about everyone goes through tough stuff often enough to provide more than enough fodder to write about and thus have a similar cathartic release when performing their music.
Metal Rules

Slayer being one of the rare exceptions since Tom Araya’s come clean in interviews (including mine) that he has a Christian-based faith yet Slayer is still one of the fiercest bands on the planet, I’d say it’s more a case of the big radar bands where this metamorphosis beacons so loudly. You can still be tough and angry sound-wise in the name of good like the Bad Brains, for example. I would say the last Maiden album is something to be proud of considering their time on the scene and if there’s a catharsis there, it’s just settling back into a rhythm with Bruce. Mental stress being alleviated does change your outlook on life and music, I’ll agree, but using Metallica’s case, I’ve come to the conclusion Cliff Burton’s input made all the difference since they’ve never been the same band Justice on after… That being said, my conclusion is it’s frequently a case of the moment in time your “edgiest” work was produced as well as the company in league manufacturing that tone which potentially dates and locks that sound unto itself where all else afterwards is subject to scrutiny.
Ray Van Horn, Jr. The Metal Minute