Why do people have to say stuff like “glad to’ve met you” in order to stay alive? What does Holden mean?

Holden  says “I’m always saying ‘Glad to’ve met you’ to somebody I’m not glad I met. If you want to stay alive, you have to do that stuff, though.” He understands that, in life, you have pleasantries to go through that aren’t always necessarily true, just to fit in with society. If you don’t want to fit in with society, you outright speak your mind and tell people when they’re being boring or that you would rather not spend the evening with them. Here, he implies that society forcers a person to be false as a means towards survival.