Einmal ist keinmal.

In one scene early on in The Unbearable Lightness of Being, Tomas is trying to decide what to do about Tereza, the waitress he met three weeks earlier in a small town, and with whom he had inexplicably fallen in love. Part of him wants to invite her back to Prague for good to be with him, but another part of him is holding him back. He spends some time recalling how he felt when he watched her, ill, sleeping on his bed, and wondering whether what he had felt was truly love, or merely hysteria.

And he was distressed that in a situation where a real man would instantly have known how to act, he was vacillating and therefore depriving the most beautiful moments he had ever experienced (kneeling at her bed and thinking he would not survive her death) of their meaning.
He remained annoyed with himself until he realized that not knowing what he wanted was actually quite natural.
We can never know what to want, because, living only one life, we can neither compare it with our previous lives nor perfect it in our lives to come.
Was it better to be with Tereza or to remain alone?
There is no means of testing which decision is better, because there is no basis for comparison. We live everything as it comes, without warning, like an actor going on cold. And what can life be worth if the first rehearsal for life is life itself? That is why life is always a sketch. No, "sketch" is not quite the word, because a sketch is an outline of something, the groundwork for a picture, whereas the sketch that is our life is a sketch for nothing, an outline with no picture.
Einmal ist keinmal, says Tomas to himself. What happens but once, says the German adage, might as well not have happened at all. If we have only one life to live, we might as well not have lived at all.
- page 8

The expression "Einmal ist keinmal" means literally "Once is never". Its sister in the English language is "One swallow doesn't make a spring". I thought it was interesting because I had never taken that expression to have a meaning the way Kundera describes it. I take it to mean something along the lines of "Don't jump to conclusions".
An interesting expression...
Why does something not count if it happens only once? Doesn't it make that event all the more special?
However, I do identify with Tomas' over-analytical and indecisive nature. For me, life is like one big choose-your-own-adventure novel, and depending on what you choose, you might end up in an entirely different situation. (I suppose I may now find myself in a heady conversation with somebody about predestination versus free will. That is for another day.)
And though I do believe in the afterlife, I have never once considered this life to be a dress rehearsal for the one to follow. My thoughts on the afterlife are not as yet very well thought out, but I think it may suffice to say that I believe life after death to be more of a continuation of our life on earth, rather than a second go-through.
Anyway, I liked this passage because the way Kundera described Tomas' indecision and then his frustration with his own indecision reflects the way I sometimes feel. But I do not then conclude that a life lived only once is not worth living at all - no, quite the contrary. Because we have only one chance to live this life, we need to pick the grandest adventure of all, making the best of every situation, making the most of what we have. Sometimes things that happen just once are more than enough.