Selene Winchester never thought she would actually lose her father, or her brother Dean. But she has, and when Sam calls her and tells her to come to Bobby's house she obliges. What she finds there is something unexpected, and when Castiel turns up to explain why and how he's unexpected too. 

Castiel is an angel. He follows his orders and never disobeys. That is, until he meets Selene Winchester. The way is makes she makes him feel is something he has never experienced before. While Castiel helps Selene to understand that not surprises can be good, great even, she helps him to discover just as much. Except, Castiel isn't learning about surprises, no, Castiel is learning that you don't have to be human to have humanity.