Tomas loves Beethoven because Tereza loves Beethoven, so he has no idea about the story behind “Muss es sein? Es muss sein!” Apparently, a man had owed Beethoven some money, and when Beethoven reminded him of this, the man said, “Muss es sein?” To which Beethoven responded, “Es muss sein!” When the exchange became the quartet, it wasn’t funny anymore and became “'der schwer gefasste Entschluss’ (the difficult or weighty resolution).” The saying went from light to heavy, or positive to negative, like Parmenides would say.
Kundera repeatedly returns to the “Muss es sein? Es muss sein!” motif in respect to Tomas. Tomas uses Beethoven’s “weighty resolution” to describe something that is out of his control, like his love for Tereza. Remember, Tomas said “Es muss sein” when he left Zurich for Prague. Here, “Es muss sein” is seen as heavy, or negative, but Tomas later realizes that his womanizing (something that makes him light) is out of his control, too, and is also “Es muss sein,” or essentially heavy.
When Tomas starts his job as a window washer, he experiences some initial shock. Soon, however, he begins to enjoy himself. It is like a vacation. He doesn’t care about what he is doing, and there is no stress. He even goes back to his bachelor ways and starts having affairs again. He walks around Prague like he is 10 years younger. He washes public as well as private windows, and when his former patients find out what he is doing, they begin to request him. Tereza works the night shift, and the only time they see each other is at breakfast. Tomas has all day to himself to do whatever he wants.
Again, Tomas doesn’t seem to have any control over his womanizing, and he slips right back into his bachelor ways. Even though Tomas is a window washer, his former patients still seem to respect him, for his station as a doctor and his refusal to conform to the regime’s demands. Tomas appears almost happy in his new life, which, interestingly enough, has very little to do with Tereza—even though she was the reason he moved back to Prague.