The third nightfall was less impressive than the second, which mirrored a rule Tris had already identified; new emotions devalued, going from intense… - Jon Courtenay Grimwood

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.

Inside every adult was a child, or so it is said. Professor Petra Mayer was different. Inside Petra Mayer was an impossibly beautiful, barefoot adolescent who wouldn’t have been seen dead giving her inner child the time of day.

"That's impossible," said Tris.
"Most things are," Luca said, "if you think about them for long enough."