This is something either old schoolteachers or prententious
slappers use.
To be fair, not everyone who uses this phrase is either or, but sadly most of the time this is said with a certain
intonation to show false delight in activity, etc that the other speaker may have just named...basically a more subtle way of saying "oh the things you get up to are so quaint and CUTE!!!"
Person 1: "How was your weekend,
sweetie?"
Person 2: "Well mainly I just stayed home, though I went
paintballing with my friend
one day."
Person 1: "Oh how fun!!!!!"