She was all that mattered to him. She, born of six fortuities, she, the blossom sprung from the chief surgeon's sciatica, she, the reverse side of all his "Es muss sein!"-she was the only thing he cared about. (5.14.17)
Tereza is more important to Tomas precisely because she was born of six fortuities. Because she came about by chance and not out of necessity by es muss sein, Tereza and his love for Tereza are voluntary. Tomas chose her in a way that he did not choose his desire to womanize nor his medical profession (both of these were part of his es muss sein and are therefore beyond his control).