My dog was always puzzled by my natural set of reactions to things too.
She was adopted by me when she was two years old, so she already had a lot of experience with other people. And she was so completely standard-non-autistic-social and so tuned in to those particular mannerisms and stuff.
She was nervous around me because I was unpredictable. If I did my best to exaggerate my emotions and stuff, she responded a good deal better. This mirrored my first experiences raising a puppy, where a psychologist actually taught me the intonations/mannerisms/etc associated with "normal" people, as a way of teaching me how to communicate with a dog who was baffled by my natural behavior (and who would not listen to me at all unless I put on an act for her).
And the newer dog was no exception to this. It actually got exhausting, because the moment I was no longer able to sustain dog-type emotional expression, she'd get nervous (she was a very nervous dog in general, anything she considered out of the ordinary, frightened her, but she utterly loved people and other dogs).
The only part of my natural behavior that she understood right away was hand-flapping, she'd wag her tail in response.
But definitely it's easy to tell if a dog is nervous once you know what to look for because of how they act. It's not mostly on their face. I've read that for most people, reading dog emotions is quite possibly programmed into their instincts, because humans and dogs have evolved together for so long. (It certainly was never instinctive for me, though, I had to be taught dog expressions one by one. Cats were easy though.)
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams