I believe it's also a defense mechanism used against predators approaching from above. It rolls onto its back to get its claws into whatever is swooping down onto it. I saw footage of a hawk doing something similar against another hawk on a Discovery channel program.

Their humor/laughter is their defense mechanism. Add with a nice shot of Adrenaline and Wartime stress... you'd have the same reaction. :-)

Because it's too cute! No matter how grumpy you are or how bad of a day you're having, one look at these two and it makes you smile. All babies are genetically born to have cuteness as a defense mechanism.

(Source: Discovery Health)

Man why don't Humans have cool defense mechanisms like that?