The Passive-Aggressive Primer - an ongoing series

Person: "I'm getting an ice cream."
P-A Person: "Must be nice."

~~~

It's all in the inflection and choice of words.

I mean, one could respond to the afore-mentioned ice cream purchase by saying "Isn't that nice!"

This comes across as "An ice cream sounds lovely, I hope to have one as soon as I have the opportunity to do so as well!"

It seems passive-aggressive to respond to the afore-mentioned ice cream purchase by saying "Must be nice!"

This comes across as "Well, aren't you lucky, and I wish I could have one if I were not screwed by life and unable to digest dairy without puking, thanks for bringing it up."

"Isn't" is inclusive, while "Must be" implies the person saying it is somehow restricted against their will and desire.