Abigail Breslin's character is literally drifting away from her father in the film - she's dying. She's rotting from the inside out, and he is forced to deal with letting her go. She tries playing on the swing set, like she would have done as a child, but falls to the ground in failure. She's no longer able to do things a child would do.

She goes out with friends her age, and in a rite of passage to adulthood, kisses the infected boy. He, like her, is also dealing with "drifting away" from his parents.

Arnold is constantly grappling with losing his daughter, finding it impossible to figure out a way to let his little girl go.

Both Arnold and his daughter share the grief of losing wife and mother respectively. In addition to drifting away from each other, they're both struggling with letting go of the person most important to their relationship.

At the end of the film, she gives her father a goodbye kiss on the forehead. She then positions herself on the edge of the roof where she envisions her mother. As she steps off the roof, she lets go of the grief over her mother, and becomes free of childhood. She becomes an adult.

Of course, this is just my interpretation!