Discord has heard Celestia sigh (which is funny). He's heard her growl (which is funnier). On a good day, he can even make her shout (which is hilarious).

But he's never heard her laugh. Until now.

Set during/directly after the events of "Make New Friends but Keep Discord", Discord sees a side of Celestia he never thought existed -- and has no idea what to make of it. The resulting discussion with Twilight and Fluttershy only annoys him further, as his frustrations with Pony society come to the fore.