That's a good one Sharon. Many things, but humour is incredibly hard to pin down and understand. - For one, I find strained social interaction between long suffering loved ones funny. There are many instances of this in lots of classic sitcoms, and the majority of them work because of this, along with the pathos, and not neccessarily because of the jokes. - The verbally battered and down-trodden Else, wife of Alf Garnett, brilliantly played by Dandy Nichols. - The get-on-my-tits relationship between Harold and Albert Steptoe. - Delboy's continual put-downs on Rodney.
Anyone who has grown up in a close family, with brothers and sisters, can relate to these strained situations, but in the best of these comedies, as in real life, it's always obvious that there is a lot of love and affection there.
I also find watching toddlers incredibly funny. - I was sat in a lovely sunny beer garden today after a pub-lunch, and myself and the other half were watching (and laughing) at a little boy who was sat in a feeding chair playing with a plastic toy mobile phone. - The baby boy couldn't actually talk, but he kept on putting the phone to his ear and shouting 'HELLO!', obviously because that's what he had seen his parents doing. - The boy kept saying this for what seemed like an hour, in his own little world. - Everyone who was sat there found it amusing.