[The Might]

I personally also saw Galadriel's "Use well the days!" advice right before Celeborn's statement as a foreshadowing of what was to come.
When she tells him that it is clear that he wants him to keep in mind that he is after all mortal and that there is a limit to the things he can achieve in the world. As such, Galadriel wants him to do his best in the time that remains.

Celeborn afterwards, also taking Galadriel's advice into consideration thinks about the question of mortality and wishes Aragorn to suffer a better fate, perhaps here also thinking about the gift of death.
And of course he also wishes him that his treasure, Arwen, will remain by his side until death and beyond. And this also truly happens when Arwen goes to Lórien to willingly give up her life.