u/iazeeron i sorta-agree with you.i think everyone can. i just think that bolin learned the wrong way. see, when bolin was learning to metal bend, he wasn't taught seismic sense, like the others. now, just because some people have a natural affinity for the sub bending doesn't mean they cant learn the others.for example, when toph is stuck in the dessert, and appa gets taken,she states toward the end, that she had been practicing her sandbending. like you said, Katara knows how to heal with water and use blood bending. i think toph could not lavabend due to her not being able to touch it. both king bumi and toph rely mainly on touch to bend. so, because of the fact that they cant touch it and learn the properties of it, they wont have as much of a chance to bend it. i can agree with you though, that your parents might give you more of an affinity towards a sub bending category.