Yeah, it's extremely common. Pair that motivated sentiment with a kid who's in the diagnosable age range (or an adult), and a diagnosis is almost guaranteed.

Most mental health diagnoses are dependent on subjective reporting of symptoms and history, so if someone - or their parent - want the label or have otherwise already convinced themselves that it's true, their self-reported history is going to be colored by that, and most clinicians are going to err on the side of the treatment that the client wants.