When I'm trying to get something done I don't want a bot telling me what a splendid day it is. I could be 7 layers deep into some rabbit hole with an entire stack of things in my head. Just give me a concise, accurate response.
In general, cutesy UIs are fun only the very first time you use them. But they’re a liability in the long term because once the novelty of the cuteness wears off, you risk really annoying your users, and especially in high-stress scenarios.
Also: I just realised that the Avengers would never happen if Stark just realised that launching Jarvis as a product would take the world by storm :D
Personally I try and think of "signal to noise ratio" and also respecting the recipient's time when communicating with them.
Of course it's not good to be terse to the point of being rude.