"But she did not go.  Either she could not find the time, or she could not fine the energy, or it seemed a great distance to Market Square, or she remembered that on her own she was in danger from Wizard Howl - anyway, every day it seemed more difficult to go and see her sister.  It was very odd.  Sophie had always thought she was nearly as strong-minded as Lettie.  Now she was finding that there were some things she could only do when were no excuses left." 
-Howl's Moving Castle by Diana Wynne Jones