I think that Dude is onto something here-- a lack of INTRINSIC motivation to do what others wish that we would do.
KWIM?
I think that the reason why in MoN's case, making the decision was the appropriate one is that it was not a top-down, authoritarian move-- it was "I've heard you, but here is why I think that my plan is better than yours right now..."
I wasn't intrinsically motivated to do much that the adults in my life wanted me doing at 12-18yo.
I only BECAME motivated to achieve at levels more commensurate with my ability once I was in college and: a) the work got harder-- honestly, I started in a major that wasn't going to work out that way for me... BOY am I glad that I switched after taking a supporting course in another subject, b) it got cool to be a smart chick, and c) something just "clicked" in my head-- I wanted it. I hadn't before-- or I thought I wasn't worth it, or something. It was all tied up in my perfectionism, too-- so if I hadn't tried, then I couldn't chalk it up as a genuine "failure" in my head... lack of "failure" was my prime directive.
