I have noticed with ds that if he is laughing about something and I touch him that it makes him laugh louder and longer.
I tried my theory this morning whilst watching Despicable me, as he was laughing I firmly touched various parts of his body and he would go off into hysterical laughter almost as if I was really tickling him. he was still focused on the TV not on my touch. After each prolonged laugh he would then say "is that funny" He always asks that when watching something he finds funny.
He is tactile defensive and gets himself into bother at school, accusing dc of hurting him when they have brushed past him. It must all be linked somehow.