Formal vs. Casual
You don't act the same way when you're with friends as you do when you're with strangers—neither should your typefaces.
If you think of typefaces as representing you, then you're on the right track.
When you're with friends or family you act more casually than you do when you're with business associates. With friends you're more personal, informal, sometimes even goofy-you long ago stopped trying to make a good impression.
With business associates you're more reserved, professional, official-you're always trying to make a good impression.
You're the most formal with complete strangers-you're trying to make a good first impression (otherwise they might find out who you really are and have nothing more to do with you). Right?
Fonts are the same way--they present a subtle “first-impression” to your readers, whoever they are, and whatever you want to say. They may not consciously notice the font, but they will subconsciously notice, and respond--emotionally.