Why Graham Greene traveled to Africa, Mexico, Vietnam

Reading Graham Greene's autobiography A Sort of Life -- a cheap edition I got used for a few dollars -- I got to the part where he talks about a "war on boredom" that he began to fight, during a depression of what he later understood to be his manic depression condition. As part of this war on boredom, he engaged for a season in playing Russian roulette.

The first time, he was rewarded with "an extraordinary sense of jubilation" and of his renewed life's infinite possibilities. But when he repeated the experience later -- he describes having done so several times during the fall the year when he was nineteen -- he found the exhilarating effect of the experience lessened each time. Finally, when playing Russian roulette no longer had the power to give him even an adrenaline rush -- which he frankly describes as a "drug" he had come to enjoy -- he gave up doing it.

But the interesting part is at the end of the chapter when he says that this practice of using what he calls recklessness to fight his "war on boredom" is what underlies his subsequent journeys abroad:

A kind of Russian roulette remained too a factor in my later life, so that without previous experience of Africa I went on an absurd and reckless trek through Liberia¹; it was the fear of boredom which took me to [the Mexican state of] Tabasco during the religious persecution², to a léproserie in the Congo³, to the Kikuyu reserve during the Mau-Mau insurrection⁴, to the emergency in Malaya⁵ and to the French war in Vietnam⁶. There, in those last three regions of clandestine war, the fear of ambush served me just as effectively as the revolver... in the lifelong war against boredom.

The references are, of course, to Greene's later books:
(1) Journey Without Maps -- actually this journey, which he undertook with a female cousin, was done on behalf of the British secret service, to assess whether Axis powers had a foothold in Liberia.
(2) The non-fiction The Lawless Roads and the novel The Power and the Glory
(3) A Burnt-Out Case
4 and 5 -- No book that I know of, but that's probably just my ignorance.
(6) The Quiet American