The phrase "life's best gifts are often unexpected and often come as surprises" fits the story of Anne of Green Gables well. Matthew and Marilla are both older and unmarried. They are brother and sister and they share a house together. They decide to adopt an orphan. They choose to adopt a boy who can help out with the chores around the farm.
When Matthew takes his wagon to the train station, he does not find a boy who can help out with the chores. Instead, he finds a skinny, red-headed little girl. Not knowing what to do, Matthew takes Anne back to Green Gables. When they arrive, Marilla is shocked. She thinks that the whole situation is "a pretty kettle of fish." She is annoyed by the mix-up, because she had specifically asked for a boy. Marilla has no intentions of keeping Anne.
Marilla soon warms up to Anne. She starts to have conflicting feelings about the child. She wonders if she should let Anne stay. The intention had been to get a boy to help Matthew with the farm work. Anne will not be able to do that. Marilla debates the issue. Should Anne stay even though she is not a boy?
When Anne is faced with a life with the harsh Mrs. Blewett, Marilla makes her decision. She decides to let Anne stay at Green Gables. Anne becomes like a daughter to both Marilla and Matthew. They cannot imagine their life without her.
This conflict of what to do about Anne is one that goes perfectly with the quote you provided. Marilla had no intentions of adopting a little girl. Anne's arrival was completely unexpected. In the end, Anne's arrival turned out to be one of the best things in Marilla's life. Marilla struggled with an inner conflict about Anne. She did not know if Anne should stay or be sent back.