"Are you happy?"
I got no answer for "why", but I do have something that may make it easier to remember:
A proper name is (usually) used to refer to a third party being thought of as a single entity. The pronoun "ia" is likewise used to refer to a single third party (neither the speaker nor the listener).
You and I are already talking, so either of us can be the subject of a sentence without needing an introduction (‘o). "Kaleo" and "ia" do require an "‘o" if they're going to be subjects.
It's a mnemonic device. They're meant to be stupid. It makes them easier to remember.