Washoe, a chimpanzee, was taught sign language. When one of her trainers returned after a while away Washoe was initially dismissive of her, but when the trainer signed “My baby died”, Washoe signed “cry” and mimicked a tear running down her face.
Crying when you are happy is your body’s way of neutralizing your emotions so you don’t become overwhelmed with feelings. Responding to positive emotions with negative reactions (or vise versa) helps restore your emotional equilibrium.
If a child wants something bad enough, they can momentarily forget how big their body is compared to other objects. It’s called “scale error” and it can cause them to think they can fit into toy cars, for example.