Genetics could have been one reason.
RFG was not the kind of the person who would seek medical, or psychological, help. He had some doubts that his brother would commit suicide, despite Roy having a long, and documented, history of bipolar disorder. Sloane said, "He wouldn't even take an aspirin."
I looked at the Sharon Comitz is one example. RFG actually said that because Comitz could plan, she didn't have a psychological problem. http://content.time.com/time/magazine/article/0,9171,149520,00.html
The one time he was sanctioned by a judge, it was for an attempt to get a physician to withdraw as an expert witness. http://www.collegian.psu.edu/archives/article_04049209-3057-5e54-9091-c17e93cd66b9.html
The pattern is there.
There is also the possibility of his financial situation not being what he wanted it to be. Assume that the reason for his low assets was bad investments. That could add to it. I don't think that is the case, but I can't rule it out.
JMO but if he had planned on retirement and his retirement wasn't enough for the life he had planned he could have been panicked and in despair. JMO again but I still doubt he had a tidy cash reserve somewhere that he felt he could live on the rest of his life. Maybe the appointment he had was with himself for suicide. Men are planners (in my experience)when it comes to suicide, as I've seen many of them. If he killed himself it wasn't an impulse.