If not genetics—then what?
Their famous father was married four times. There was a bitter struggle over custody of the boys when they were small, resulting in the state taking the boys away and putting them in a facility for a number of months that either Robert or David described as "like prison." There were two stepmoms going forward from there.
People with Irish genes may also a genetic propensity for low brain serotonin and depression. In spite of the glamorous surfaces and good looks, there was probably a lot of private anguish. And Hollywood isn't exactly known for sustaining relationships with anyone, much less with the Lord.
RIP.