Immortality had been perfected, at least in its non-biological forms, but insufficient attention had been paid to the boredom of eternity and the corrosive nature of the ratchet effect which demanded ever sharper, stronger and more intense sensations to maintain something like the same level of satisfaction.
Living forever turned out to be much like long-term sex, psychologically tricky; which was why what killed the original colonists was not hardship but boredom.

Chapter 10 (p. 71) - Stamping Butterflies (2004)