Could be something that plays a role in affinity. Like wizard-Molly, who likely has always been sensitive.
But what would you think is for example Carlos' character trait, which causes him to be good in water magic?
Listens to Wind is a healer.
Who else have we got?
Harry would be fire and force, but he also says about himself, that he is really good at finding things. Is this hubris? Are all the other wizards good in finding things, too, but just don't make a fuss? I mean he has turned it into a profession by becoming a PI.
I believe Harry's talents tend toward creating: focusses, magic items and I think Names.
All the other stuff, the fire, the force, are just him having a lot of power to let those loose. You put a lot of power in your fire and force and the result us a bigger kaboom.
Or maybe it is tradition. Justin taught Harry to use fire very early. So he has a lot of practise.
Someone taught Carlos the use of water magic. So logically this is what he does best.
Could be the personality or could be personal taste.
Or it is what zodiac one is born...
So far there is nothing in the books about it, but I don't think it is either the one or the other, but more a combination of everything.