When he returned to Prague from Zurich years earlier, Tomas told himself, "Es muss sein!" about Tereza (i.e., that he was meant to be with her). But he began to doubt this after he crossed the border.
Lying next to her in bed, he realizes he was drawn to her only "by a chain of laughable coincidences" (5.7.1). He decides that his love was not es muss sein.
But the narrator believes that there was an element of es muss sein in Tomas's life after all with his profession. His career choice was rooted in a deep-seated desire.
If we can separate man into three categories, says the narrator, we can do so by deep-seated desires that drive them to their profession of choice.
Surgery is interesting because it "takes the basic imperative of the medical profession to its outermost border, where the human makes contact with the divine" (5.7.1). God never took surgery into account, he explains.
And Tomas sensed this blasphemy the first time he operated. But this is also what attracted him to it. Surgery is his "es muss sein!"
For this reason, it seems to odd to the narrator that Tomas could so easily cast off his profession and become a window washer.
Could his decision perhaps conceal some other reasoning that explains it?