Karenin did not like the move to Switzerland. Dogs don't like change, because time for them is circular, not linear.
Karenin helped Tereza to hold on, because the dog was weaker than she, and she felt she needed to take care of him.
One day, a woman calls their flat looking or Tomas, while he is at work. Tereza doesn't know who the woman is. She realizes it could just be a secretary or something insignificant, but it's too much for her. She can't bear being in a foreign country, dependent totally on Tomas, having nowhere to turn should he abandon her.
She decides that she is too weak to live with Tomas, and so that's why she returns to Prague.