At this stage, it is too early to make grand conclusions. But we can say with certainty that [baby] laughter is a central component in early development and it is likely that our sense of humour starts to form far earlier than most theories of humour currently admit. Furthermore, there appears to be greater variety and subtlety in the sources and purposes of laughter than was previously thought...Babies can laugh long before they can talk or communicate in other ways. Smiles and laughter are not only the welcome relief that help parents (and babies) cope with the tears and confusion. They are also a shared celebration of all the triumphs and achievements in an infants’ life.
This also highlights the importance for parents and children of staying happy and positive throughout the wild ride that is parenting in the early years. Not only is shared laughter the quickest way to connect two people but perhaps the secret to happiness is retaining a childlike ability to laugh at the world. We think the shortest answer to the question why do babies laugh is ‘because they are happy’.— from The Science of Baby Laughter by Caspar Addyman and Ishbel Addyman (2013)