Things that motivate Benedick are his love for Beatrice. Though out the story Benedick realized that they still both love each other but they aren't sure. Another thing that motivates Benedick is that he wants to have freedom. He show this in the story when "Go to, i' faith, an thou wilt need thrust thy neck into a yoke, wear the print of it, and sigh away Sundays." This tells the reader that he doesn't want to be with a woman because he thinks that his life would be ruined.
Benedick changes though out the story in a couple of ways.In the first part of the story he didn't think marriage is a good idea. Then after he new that Beatrice likes him he changes his mine on how he sees marriage. (I can't get a quote because I don't have the book.) Now he thinks that marriage is something that all people should do.
In the story Benedick impacts the story in a couple of different ways in the beginning of the story he told Claudio don't get marriage your life will be ruined. Towards the end of the story he changes him mind on marriage and thinks it is a good idea to get marriage. The theme of the story is love and now Benedick agrees with this theme.