tnlibrarian right here on this board recently: great husband, great kids, great job, originally, and suicidality extraordinaire. It just happens more rarely but sure there is endogenous illness not exacerbated by circumstances. And she recognized herself that anyone else in her situation would have been happy... but she was suicidal. No childhood trauma, either. No exogenous explanation.

Or my friend here: three great daughters, academic husband who does not mind whatever my friend is doing, be it camping with her friends, going to the Stone Age seminars with the children, blogging, defending the rights of tortured prisoners, whatever she wants to do. Enough money from his income. Everything great. Remarkably good diet, remarkably. She exercises and is thin and petite at 50+. She published a good progressive book about fifteen years ago. Basically, an enviable life situation. Still, the usual: anxiety and depression.