I can hold it back when I want to/need to, but most of the time, as most others have said, it's simply who I am. I've always woofed and growled and rocked back and forth when my tail's wagging. I usually don't notice it. Heh... come to think of it, the other day a collegue walked into my office and skritched my head to get my attention (I was wearing my headphones with my back to the door)... I know for certain I let out a murr before I realized what I was doing. Fun times.
As far as my friends go, we're all geeks of some stripe. We comment on how funny it is to watch a "scientific show" like CSI and point out how there's no way that they just did whatever procedure in a minute when it takes at least 2 days. *shrugs* I have a feeling that if I said: "You know, I really think I'm a dog trapped in a human body" that they would probably put milkbones on my birthday cake and go with it.
-Kai.