Why does Sam kiss Charlie at the Christmas Party?

At the party, Sam looks at Charlie and asks him if he has ever kissed a girl. When he responds that he hasn't, she tells him that her first kiss was from one of her father's friends - which is the first time that Charlie realizes that Sam has been sexually abused. Sam asks Charlie to forget everything for a moment because she wants to make sure that the first person who kisses him loves him, and they kiss.