"Come," I said, with decision, "we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed
This line is in a sarcastic tone. Sarcasm is a form of verbal irony. Irony is when what is said or written is actually the opposite of what is meant. The narrator does not actually believe that Fortunato is to be respected or admired. He doesn't actually think Fortunato's health is precious as his intent is to kill him. Also, he doesn't want them to go back. He has everything planned out for Fortunato's entombment farther into the catacombs.