MSNBC Freelancer Patrick Enright uses Ben Stiller to put forth a pretty bold idea of why people start bleeding very bad work :  

"Some wags might be tempted to attribute your career implosion to bad management or drugs or Scientology," Enright comments, "but the real culprit is much, much more sinister. Something else happened in 2002, the year your movies went south so drastically: Little Ella Olivia, your first child, was born. Coincidence? I think not. And you didn't stop there; No. 2, Quinlan Dempsey, came along in '05.

Yes, I blame the children -- it's a well-substantiated, scientifically tested theory that having kids drains the talents of great comedians, leaving them hollow and laugh-deficient, capable of and interested in only fart jokes and the occasional animated movie.

The once-hilarious Eddie Murphy went through exactly the same decline, starring in his first major bomb, Harlem Nights, the year his first kid appeared on the scene. Whether it's caused by a hormonal shift that kills any sense of The Funny or springs from a desire to create movies that please the young spawn, the trend is career-fatal.

I would go farther and say that for many artists, feeling happy and settled and soothed isn't precisely "bad" for their creative output, but it doesn't seem to exactly help either. This may sound deranged or masochistic to some (and I'm not saying it applies to each and every actor, comedian, writer and gallery artist) but some artists feel they need to be on the edge in order to keep themselves sharp and focused -- to pick up on those shortwave transmissions that tell them what's next and where to go."

Personally I somewhat agree. I watched a background actor (who shall remain nameless) stop leaving the house after his kiddo was born. He not only moved from LA, he ran from LA and the biz. Natural but it does make me melancholy.