soft spot vs weakness

soft spot

noun

  • A sentimental fondness or affection. 

  • A point of vulnerability in a defence. 

  • A fontanelle. 

weakness

noun

  • A special fondness or desire. 

  • An inadequate quality; fault 

  • The condition of being weak. 

How often have the words soft spot and weakness occurred in a corpus of books? (source: Google Ngram Viewer )