Also, please explain why you think it's important for parents to push children when they run into difficulties. I agree that there is a certain amount of basic knowledge, which everyone should acquire whether they want to or not (I've even found benefit from statistics classes). But we're talking about passions -- that is, recreations, not basic knowledge. I agree that encouraging someone to get through difficult spots -- to reassure them if they fear failure, to provide superior knowledge and skills on an as-needed basis -- is a good thing, but the whole point of a passion is that it's something that the person in question _wants_ to do.
So the child can learn that the limits of their ability are beyond the limits of their self-perceived ability.
We're talking about passionately applying oneself to an activity. This does not limit the locus to recreation IMO.
Additionally, kids have parents because they're extraordinarily unskilled at balancing the long-term perspective against the here and now (granted, many adults aren't much better). I find it hard to believe that just letting your kids do whatever they want is really the path to greatness for most kids. I think parents should help kids explore their passions but it seems there's a balance to be found where you also don't let your kids give up as soon as it gets difficult. My parents did that too often, and my wife's parents pushed her through the rough spots. I won't go into the whole results, but it's clear to me which is superior.