Because of her inner conflicts

I believed Sr. Luke when she told her father than Jean (the ex-boyfriend) was not in her thoughts and when she told her superior that she was not in love with Dr. Fortunati, but simply admired his skill as a surgeon. I also think The Ex-Nun's Story would be fascinating. How difficult it must have been to rejoin society, over age 40, after 18 years in a cloistered religious community -- at least that was the timeline of the story's inspiration, Marie Louise Habets, https://en.wikipedia.org/wiki/Marie_Louise_Habets.

Hulme’s 1966 autobiography Undiscovered Country describes Hulme and Habets’ first meeting in 1945. Both were volunteers with the United Nations Relief and Rehabilitation Administration (UNRRA), an international project working to resettle refugees and others displaced by the war. Hulme recounts that, at a training camp in northern France, she became aware of a Belgian female colleague who spent most of her time asleep. Even when awake, the woman, a nurse, was taciturn, solitary and preoccupied, almost antisocial. In time, however, the Belgian nurse revealed herself as a diligent worker, a good friend, and a woman with a secret: she had just left the convent after 17 years of struggle with her vows. She felt burdened and depressed by a deep sense of failure.