When animals show teeth, we tend to think of it as a threat. But this facial expression is actually a smile!
It is the reaction of a male black crested macaque to a female backing up to him and presenting him with her pink, swollen behind, a typical way for a female monkey to invite a male to mate with her. In response, the male sends her a big smile to let her know that he is in a friendly mood and welcomes her approach.

Tangkoko Nature Reserve, Sulawesi, Indonesia.