It is a common thing among those who grow up being praised for their intelligence. If one comes to base their self-worth on their intelligence and believe it is a static quality they were born with, struggling and/or failing could demonstrate that perhaps they weren't as smart as people gave them credit for and thus decrease their inherent self-worth. They quit things if they don't immediately excel at them or just adopt the "slacker" role and put in minimal effort, brushing off their failures as merely the result of not really caring about whatever it was they were attempting. "Eh, I could have been good at [x], but it bored me" or whatever