From The Heretic Knowledge Vault
A state of wistful looking-back-over-one's-shoulder felt by an elf when his/her inamorata dies before becoming tiresome -- as elven inamorata inevitably do (elven relationships just aren't forever, mainly since elven lifespans are). According to Sarine, it's celebrated by elven playwrights, etc., as just about the best emotional state an elf could aspire to. Since it's something she herself has a degree of (apparently unacknowledged) experience with, courtesy of the early death of her human mate, and she apparently considers it (or at least the "best emotional state" part of it...) to be a crock, one can conclude either that Sarine is not your garden variety elf, or that aetern desiderium isn't all the poets crack it up to be.
Note that the Epilogue reveals that Sarine eventually gets a double dose of this dubious affliction.