When she hits adolescence, though, her enthusiasm starts to wane a bit. It's not cool to be so into your parents' weird antiques. Objectively speaking, she doesn't actually care for the Impala that much. She's very into Causes (she wants to fulfill Uncle Sammy's old dream and become a lawyer, with the goal of helping victims of the supernatural- e.g., people who were possessed and aren't actually responsible for the crimes their bodies committed but who will never be able to prove that to a jury), and that includes being green. If she were buying herself a car, she'd get a hybrid.
When she and Dad argue (as teenage girls and their fathers are wont to do), insulting the Impala is the easiest jibe and also the most fun. Watching him huff and puff indignantly when she calls it old and inefficient and waxes poetic about the merits of modern cars is always a treat. Inevitably he'll threaten to disinherit her and give the car to her younger brother Jack (short for John Robert Castiel), to which she just replies, "I'd like to see you try!"
Whether or not she ever plans to actually drive it, the Impala is hers, and it's going to stay that way (fortunately, Jack's pretty complacent about it; she's promised to teach him how to drive it when he gets his permit). In high school she ends up driving it more often than Dad, green proclivities aside (although the snark and the threats to disinherit continue flowing freely), and when he suggests she take it to college with her, she doesn't hesitate.
("Are you crying?" she asks, at the end, when everything's unpacked and there's really no need for her family to still be in her dorm room.
"No." And then a moment later, because his eyes are undeniably red, and his voice is undeniably hoarse, and there might be a few tears trickling, "Maybe...I'm just gonna miss my baby."
"Yeah," she says, also hoarsely. "Me, too. I mean." And then he's hugging her too tightly to speak any more.)
Whenever she visits home and the weather allows, she and Dad wash it together and catch up on classes and work and boys ("Any kneecaps I have to break?" "Dad!").
They still snark, but she more-or-less stops acting like she doesn't want it, and he more-or-less stops threatening to find someone who will appreciate it more. Instead, she finds new ways to use their beloved car to send him into tailspin.
"Dad, my boyfriend's coming over for dinner. Be nice or else."
"Or else what?"
"I'll paint the Impala pink."