I like it when people speak in a deeper voice than usual, put on a deep voice, I suppose you'd say. It makes me happy, makes me feel a little like Kojak on a good day.
I suppose it's because it makes someone sound a little more vulnerable; it's opening yourself up to something. You simply cannot speak in an unusually deep voice to just anyone. Try ordering a coffee somewhere that way. Or call your boss. Or return a DVD.
Anyway, it's especially cute when someone with a higher voice (mostly females) do it, because there's the sense of a natural disadvantage. And even the biggest haters of Barry White had to admit - he sounded pretty freakin sexy.
Making your voice higher than the norm is, however, generally speaking, annoying.
Also, I've found that if you ask someone to sing a song in a deep voice, they'll generally choose Swing Low, Sweet Chariot.