My personal opinion on aggressive vs. defensive and bitey snakes:

I consider an aggressive bite to be a feeding-related bite- the snake thinks you are food, it's hungry, it strikes and maybe even constricts you. This kind of bite is avoidable, just feed regularly, make sure your hands don't smell like snake food, etc. The snake can't really be 'trained' out of this type of biting- how much you handle the snake doesn't affect it, because the snake isn't striking to make you go away, or because its scared and doesn't want to be handled, but because it wants to eat and has mistaken you for food.

The other type of bite is a defensive bite, where the snake is either scared or territorial or, for whatever reason, just doesn't want to be handled. Some snakes can be 'trained' to reduce or even eliminate this type of biting with plenty of calm, gentle handling. However, with some species of snakes, such as notoriously nippy Green Tree Pythons or Amazon Tree Boas, it may be impossible to eliminate defensive biting, no matter how much you handle them, and excessive handling may not even be a good thing for these types of snakes because it stresses them.

Again, all of this is just IMHO and based on my personal experience.