Well, Celestia's been around for over 1000 years. It would be hard to find something interesting. Wonderbolt show, Been there, saw that a few hundred times. Read a book, she had probably read every book in existence, twice. Watch a cat video; that might keep her occupied for a few days. But, A good practical joke never gets old, because even though you can pull the same prank on a bunch of different ponies, it never gets old, because they would all react differently.
Thus, when Humanity learns how to live forever, society will devolve into constant trolling and practical jokes. Look at today, where life expectancy is twice what it was 100 years ago, and look how much troll-ish humor is out there.