When my Boston Terrier, Boogie, greets an unfamiliar dog, his ears are up and his neck is stiff. When he greets a buddy, his ears go back. Boogie is constantly giving signals which let me know if he is comfortable with another dog and if he is in the right frame of mind to be social.
Like all dogs, Boogie constantly expresses his emotions through body language. Humans just need a dictionary to read the signs. Below, I’ve drawn out a a range of emotions from anxious and stressed, to happy and sociable.