It's also possible he got tired of dealing with it.
"The longer people are battling it, the more helpless and hopeless they feel," said Kaslow, adding that when it gets to feel too overwhelming people need to reach out for help.
"It's so damn hard for the victim to be convinced they are ever going to feel better," legendary talk show host Dick Cavett told ABCNews.com.
Cavett, who publicly struggled with depression for many years before getting it under control with a "modest amount of a couple of meds," said it's especially tough for people in the show business' spotlight.
"It's full of anguishing aspects if the mental landscape is not terribly healthy to begin with," he said.
Koenig apparently was fed up with being viewed as "Boner" or Chekov's son, Miccio said.