It’s a reaction that scientist theorize as a means of emotional self-regulation; a means of compensating when one emotion starts to get out of hand.
We experience this when something is unbearably adorable, and you feel like you have to squish it; or when you get so happy, that you start to cry, or so mad that you break into hysterical laughter.
We are pretty awkward with our emotions from an animistic perspective.
We recommend the book The Naked Ape by Desmond Morris. It explains what laughter really is, how it evolves from crying to laughter in the first months of life. That book may be eye opening experience.