A dozen of quotes in IMDb but the film's tenet is left off, that lightness of being means accepting a certain lack of ultimate meaning in life, and living for momentary beauty.
Here is how Tomas explains his version:
If I had two lives... with one, I'd have her stay at my place. With the other, I'd kick her out. Then I'd compare and see which was best. But we only live once. Life's so light... like an outline we can't ever... fill in... or correct... make any better. It's frightening.
And there is a debate on freedom:
The invasion of our country constitutes a clear act of aggression against an independent country. Our Czech people had the right and the duty to fight against the aggressor. People who don't have the courage to fight with arms in their hands do not deserve freedom.
-So why did you emigrate? Go back and fight. It's easy to tell other people to fight. One day, everybody will be asked, "What did you do against the communist regime?"