The first-person narrator reveals that he has been thinking about Tomas for years. But it is only now that he sees him clearly, in the light of these reflections (namely, the novel you're reading about lightness and weight).
He sees Tomas standing in the window of his flat and looking across the courtyard, not knowing what to do.
Tomas met Tereza three weeks prior. They spent an hour together. Ten days later, she visited him, they made love, she got the flu that night, and she stayed at his place for the next week.
Tomas has come to feel love for Tereza. He feels as though she was a child placed in a basket and sent down the river to him.
Now that Tereza has recovered from the flu, Tomas is left unsure of what to do.
Should he invite her to come back to Prague to see him again? He's afraid that if he does, she will offer up her life to him.
He remembers taking care of her while she had the flu; he's pretty sure the way he felt at the time was something akin to love. Unless he was only pretending that the hysteria he felt was love.
He doesn't know what he wants.
"We can never know what we want," interjects the narrator, "because, living only one life, we can neither compare it with our previous lives nor perfect it in our lives to come" (1.3.15).
Instead, "we live everything as it comes, without warning, like an actor going on cold" (1.3.17).
"Einmal ist keinmal," Tomas says to himself – what happens once might as well not happen at all (1.3.18).
And then we come to the intense conclusion: to live life only once, we might as well not have lived at all.