Part of it is Fluttershy's fault -- at the heart of it all, she was spending less time with him (albeit with a pony who is probably only visiting for a few days ala Gilda) and she's not a particularly *sincere* pony.
"I've never known anypony as funny as you!" She uses that line twice.
On the other hand, Discord isn't really very funny. Especially when he tries.
"What is he doing?" "I think they're... jokes?"
That's because comedy on demand is hard. It's easier to just randomly notice an opportunity and take it.
"You're the most basic of jokes."
The other part of it is, of course, Discord's fault. Everything to do with him has to be a huge, massive undertaking -- even delivering a letter. If he had any consideration for the mailponies, he would have gotten his ticket on time.
At least he's reformed enough not to actually hurt anypony. Although his definition of 'hurt' is still pretty narrow and doesn't include getting ponies Lost in Time and Space or Sent to Another Dimension.
I saw someone ask, "If Discord can just randomly banish people, why didn't he send Twilight and company to another dimension instead of messing around with the whole discording turning them gray thing?" but I think the answer is obvious -- he can't send people to another dimension unless he's really angry. The claws have to be out if he's going to tear a gash in reality.
The resolution was cute, though.
"I'm sorry that you got caught in the middle of my, um, wrath."
"Uh... I need a few minutes to clear my chakra before I can hug you from a place of authenticity."
Also... OMG CREEPY HAPPY CELESTIA WHAT IN THE NAME OF THE SMOOZE JUST HAPPENED I CAN'T EVEN --
Still, a very funny episode. 3/3