Why do humans/primates laugh/smile when they are happy, and not cry? This might sound funny. But I have been wondering why do humans/primates laugh or smile when they are happy or find something funny?

Why don't they cry when they are happy and laugh/*smile* when they are sad?
Subcortical unconscious programming originating in distance evolutionary relatives cause humans to smile when they feel happy. 

-Consciousness, Volition, and the Neuropsychology of Facial Expressions of Emotion

There are disorders like the Pseudobulbar affect which cause people to express a different emotion than what is typical like crying instead of smiling. With PBA the emotions displayed are sudden, frequent, extreme and uncontrollable.