Through Tereza, Tomas came to love Beethoven. But he didn't know the real history of the phrase "es muss sein." The narrator fills us in on the story:
Some guy owed Beethoven some money. When Beethoven asked for it, the guy reluctantly asked, "Muss es sein?" and Beethoven replied with a laugh, "Es muss sein." The composer liked the sound of it so much he made it into a motif in one of his works. (You can listen yourself to the String Quartet No. 16.)
Beethoven ended up imbuing the words with a far more serious meaning than the playful conversation in which they first arose.
(German, argues the narrator, is the language of heavy words.)
It's interesting to note that light goes to heavy. The narrator says that it could never be the other way around, though Parmenides would argue the opposite. "We no longer know how to think as Parmenides thought," he says (5.8.4).
The narrator proposes that herein lies the deeper reasoning for Tomas's decision to abandon medicine.
He secretly resented the es muss sein all his life, and longed to go, as Parmenides would have it, from heavy to light.
As evidence for his theory, the narrator points to Tomas's break with his first wife, son, and parents – his rejection of his (heavy) duty. That was his external es muss sein; medicine was his internal es muss sein.
When he first took the job as the window washer, he was in shock for a few days. But then he realized that he was on a long holiday.
He felt the blissful indifference of the lightness of it all.
His customers all knew that he was really a doctor forced to abandon his post, and so treated him with great respect. They would call up the window washing company, request Tomas, and then have him over for champagne while he was supposed to be washing their windows.
In doing so, Tomas reverts to his bachelor existence; he sees Teresa rarely, has sixteen hours a day to himself, and has freedom.