Even when laughihng and joking outwardly, inside he was constantly battling a dark cloud.

Yup. I worked on a psych unit in Boston for a year. One patient was a handsome, athletic, charming, bright, guy in his mid 20s. He had 3-4 well-adjusted siblings and parents who seemed very nice. He also had multiple admissions for suicide attempts. All the female staff were in love with this guy -- a "hunk" in their words. None of that mattered. The list of things he enjoyed got shorter and shorter until the only thing left was downhill skiing. Late that summer he parked his dad's car on the top of the Tobin bridge and walked off the edge.

So, as much as I quibble about multifactorial causes and such, I do not want to minimize the biological.