What does the narrator describe as "the most excruciating thing" (17) about Kafuku's experience of knowing about his wife's affairs?

The way in which the narrator describes Kafuku's efforts to hide his knowledge from his wife while going about their daily life together clearly demonstrates the theme of isolation. Because Kafuku is a professional actor, he had been able to pull off the "performance" (17) of pretending like he knew nothing about his wife's serial affairs. Elsewhere in the story, it is mentioned that the affairs with other men had begun after Kafuku and his wife had lost an infant child. In sum, then, Kafuku had lost a child and his wife, whom he adored, had begun seeing other men. The isolation inherent in his situation is brought back to the motif of acting running throughout the story when the narrator describes Kafuku as "Smiling calmly when his heart was torn and his insides were bleeding" (18).