And of course there''s the question of why DEATH turns Mort's life-timer over. It seems to me that may be because MORT could have destroyed DEATH and become MORT, but chose not to. DEATH thus owes him his continued existence.
*Terry Pratchett & Steven Briggs - The Discworld Companion; p. 278
Just reread this one, sorry I'm late...I got the impression that when Death turns the lifetimer over, this is a "human" decision - as Tonyblack said:
Tonyblack wrote:Death becomes more human and Mort becomes more Death-like.
He knows it is against the "rules", but decides to do it anyway, for the sake of Ysabell and having been impressed by Mort's character in standing up and fighting him.
I agree with what others have said that Mort was chosen for Ysabell's sake as well. Maybe also the name, when Death ask's Mort's name and says "WHAT A COINCIDENCE", is he saying it ironically, having chosen Mort's lifetimer himself at the start of the book?
One question I wondered about was about the original assassin, surely Mort killing him prematurely is as bad as extending Keli's life, or worse? Is the fate of the assassin explained at all later, is he brought back to life when the reality bubble sets back in? Questions, questions...