Going back to the notion of life having weight based on its recurrence versus its lightness as events occurring only once, I find something in Tomas’ reaction to meeting his grown son whom he has never spoken with since the boy’s early childhood.
Now that they were looking each other in the eye, Tomas noticed that when concentrating the boy slightly raised the left side of his upper lip. It was an expression he saw on his own face whenever he peered into the mirror to determine whether it was clean-shaven. Discovering it on the face of another made him uneasy.
When parents live with their children through childhood, they grow accustomed to the kind of similarity; it seems trivial to them or, if they stop and thing about it, amusing. But Tomas was talking to his son for the first time in his life! He was not used to sitting face to face with his own asymmetrical mouth!
Imagine having an arm amputated and implanted on someone else. Imagine that person sitting opposite you and gesticulating with it in your face! (…) Even though it was your own personal, beloved arm, you would be horrified at the possibility of its touching you! (p. 216)
Rather a long passage, but it was necessary to quote as Kundera brings us into the moment that goes beyond the political purpose of the meeting to touch upon the more human emotional response. While I see Kundera’s exploration of Tomas’ reaction that ranges from curiosity to trepidation, I am struck by the notion of life repeating itself in the act of procreation. While Tomas still feels no particular parental instincts that would change his life, Kundera instead focuses on the idea of repetition. In particular, he uses the image of Tomas seeing his face every morning in the mirror as he shaves. The mirror as a metaphor seems appropriate to the theory, as does the face to face meeting with his son, another mirror image of himself. This would appear to give his life "weight" and yet the thought upsets him. Perhaps he has not yet recognized the more unbearable lightness as contrast?
There is also what would seem to be an inconsistency in the idea of choice and its impact on the future here:
(…) Tomas suddenly saw that what was really at stake in the scene they were playing was not the amnesty of political prisoners; it was his relationship with his son. If he signed, their fates would be united and Tomas would be more or less obliged to befriend him; if he failed to sign, their relations would remain null as before, though now not so much by his own will as by the will of his son, who would renounce his father for his cowardice.
He was in a situation of a chess player who cannot avoid checkmate and is forced to resign. Whether he signed the petition or not made not the slightest difference. It would alter nothing in his own life or in the lives of the political prisoners. (p. 216)
The second paragraph would appear to be in conflict with the prior, arguing the impact of the signing of the paper on his life. Or, there’s another way of looking at it; while outwardly there would be evidence of change as defined in the first paragraph, to Tomas, even an association with his son would mean no more to him than his son’s rejection of him. This would then negate the value of choice and may in fact come back to that "lightness of being" –though Tomas does not see it yet, even if the reader may.
The Lost Children: A Charity Anthology