In Chapter 10, why do people laugh when John calls the director "my father" ?in chapter 10 of Brave New World, Linda and John come in the room....

In Huxley's Brave New World's  a return to Chapter 3 will provide clarification as in this chapter, Mustapha Mond, the Resident Controller for Europe and one of the ten World Controllers sits down on a bench beside the Director who speaks to children, who have been cavorting in sexual games.  He tells the children that the old world did not allow people to take things "easily," it did not allow them to be "sane, virtuous, happy":

What with mothers and lovers, what with the prhibitions they were not conditioned to obey, what with the tempatations and the lonely remorses, what with all the diseases and the endless isolating pain, what with the uncertainties and the poverty--they were forced to feel strongly.  And, feeling strongly...how could they be stable?

The word mother becomes obscene and dirty for these children as it is associated with pain, fever, groaning,--emotion--old age and poverty. Mond tells them,

"Our ancestors were so stupid and shor-sighted that when the first reformers came along and offered to deliver them from those horrible emotions, they wouldn't have anything to do with them....

"And home was as squalid psychically as physically.  Psychically, it was a rabbit hole, a midden, hot with the frictions of tightly packed life, reeking with emotion.  What suffocating intimacies, what danterou, insane, obscene relationships between the members of the family group!  Maniacally, the mother brooded over her children...lke a cat over its kittens...Yes, you may well shudder!"

The idea of parents is funny in a "scatological" way.  If someone calls another "Father," it is actually an deprecating joke since the Trobriands conception has done away with natural childbirth.  Nobody has ever heard of a father or mother, words whose connotations are humorous in a "bathroom joke" sort of way, and smutty.  However, there is nothing humorous about old age which is anathema to the conditioned citizens of the Brave New World. Linda's appearance is repulsive; she reminds them of the dying people at the hospital.