It's true that bending is part of us - and I learned to control my bending early on... But it's not like that for everyone. You aren't born a master. It takes years of training and hard work in order to truly master it.
Things like fire... it's easy to lose control of it and hurt others without meaning to, just because that's how fire is. It can be that way for waterbending, too, even if it doesn't burn you.