Tourists risk animal bites by misreading wild monkey facial expressions as 'kisses'. Wildlife tourists frequently mistaking animals' warnings of aggression for 'smiles' and 'kisses', leading to welfare problems for primates and risk of injury for people -- and educational tools such as 2-D images and information signs like those found in zoos or animal parks aren't effective at improving recognition, according to a new study by a team of behavioral ecologists and psychologists. Source 6d.
Monkey Attack (Hover)
These are wild animals and people forget that because they are cute. I remember not too long ago I read about some guy that wanted to pet a cute little elk. A cute little male elk may weigh in at 1,000 lbs and have a six foot long, spear-like rack on its head which it used to kill this fool. These are wild animals and wild means wild.
Indeed, you generally don't need to be afraid of most wild animals and getting quite close can be done, but knowledge is key. If you don't know what you're doing, get the hell out.
General tip, in very few cases are bared teeth the equivalent of a human smile, more often it is pretty much the opposite.