Tomas keeps up this perspective for about two days. Then he sees Tereza everywhere. He can't stop thinking about her. He feels as though she's infected with compassion and he cannot be rid of it.
So for two days Tomas enjoys lightness; but after that he is "hit by a weight the likes of which he has never known […], for there is nothing heavier than compassion." (1.15.4). On the fifth day after Tereza's departure, Tomas quits his job at the hospital in order to return to Prague to find Tereza.
The director of the hospital is offended, but Tomas just tells him, "Es muss sein."
The line is from Beethoven's last quartet, and means, "It must be." The narrator points out that this is Tomas's first step back to Tereza, because she is the one who introduced him to Beethoven's music.