Laughter releases endorphins in the brain. These pleasure-hormones have a reward feature. There is a lot of speculation of how laughter evolved- most scientists say it is what is called an 'emotional contagion' something akin to a positive cold virus! It is perhaps a way humans established social contact. Laughter can be physically stimulated - like the tickle- , socially initiated or stimulated through jokes as a cognitive response.
It seems our ancestors, when feeling settled and happy perhaps after a large family meal ( around a barbecued mammoth?) realised the benefit of laughter as a social interplay. It built closer relationship among kith and kin and had a survival benefit as groups tend to last longer and communicate better than single existence. The pleasure response laughter gives gives internal enjoyment and restores our serotonin balance ( happy hormones!)
Its all a lot of speculation using big words- but it sounds good doesn't it!!