A basic expression of emotion, laughter is a way of breathing. Normal breathing follows an expansion/contraction pattern of the intercostal muscles, and laughing is a clusterfuck of those contractions, forcing air out the body.
While you chuckle, the systems responsible for posture[footnote]’Postural reflexes.'[/footnote] are suppressed, hence why people ‘crease’, bend over and fall.
On the social side, it’s an evolution of grooming. Among chimpanzees and apes, grooming is a 1-to-1 activity which releases endorphins and creates bonds among social groups. Laughing is a similar concept, but it’s 1-to-many. That’s why human social groups are some 3x the size of the monkeys’ — ‘remote grooming’, if you will.