Please do not react to animals as humans. In our society a grin or smile is a sign of freindliness, an open-handed gesture of peace. It is a signal that indicates being receptive and maybe a cue to initiate a communication.
In the animal world the same gesture is interpretted as a sign of hostility, why?
When you smile you are showing your teeth. This could cause a Fight or Flight reaction from many animals. Especially monkeys and canines.
Same goes for Eye Contact. Directly gazing at an animals might interpret it as a hostile gesture. This is also the same with some humans, for example, many times certain males do not like how someone is looking at them, What are you looking at?!?.