Sentence 1: Janet's friend Mary had a birthday coming up. 
 Sentence 3: Janet decided to throw her a surprise party 
 Sentence 4:  She called up all of Mary's friends and invited them over 
 Sentence 5:  Mary went into the house full of people and was pleasantly surprised 
 Option 1: Mary has always been good to Janet so she wanted to do something. 
 Option 2: Mary had not been a good friend lately to Janet so she did not want to do anything for her.