Why we laugh at jokes?

This question ALWAYS puzzled me. I thought about it long and hard, for years now, and no answer. This question was the door for me to new thinking school. My only almost answer i could think of is social behavior (heard something similar once on tv). That will explain why we laugh easier when people around us, and it explain why we might laugh by just looking at other people laughing. this won’t, however explain why we sometimes laugh alone. but that rarely happens in the first place which again supports that theory. my explanation of self laughing is laughing became so engraved in us so we started to think of it as a standalone thing, not a social thing. So when we r in some level of happiness we laugh even if we are alone.

The school of thought that this question opens is there are some stuff that we do that doesn’t make sense or have a reason. we only do them because it is acceptable or recommended in the society. This relived me of many questions about behaviors that doesn’t make sense logically.

Some Jokes are funny and some are not, WHY? Usually funny jokes have some aspect of surprise or unexpected punch line, but still, why that is funny? Why not just be surprised?

this lead me to another question, do you think smiling or laughing is correlated to happiness by nature or by society? Imagine the following controlled experiment. if a kid was taught to frown whenever he is happy, this can be done by playing with him in a frowning face rather than with smiling face. and smile when we punish him, and frown when he do something good. do you think this will work? I thought about babies, they cry when they are not happy, and this start at birth so it is not an acquired skill. but this because screaming grabs more attention than smiling since crying is vocal and loud. so this doesn’t prove that smiling is by nature linked to happiness. Babies try to imitate, so they learn smiling pretty early since most people around them will have big smiley faces when they play with them or when they look at them, so babies can easily acquire smiling as a way to show happiness.

anyway, this is a theory i am thinking of, what do you think?