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human
[Avatar: The Last Airbender] Why is blood bending depicted as so hard?

I understand that blood bending may be harder than your average water bending, but why is it only able to be used by very powerful water benders, at one point it could only be done on a full moon, water is water, right?
gpt
Some might say it's because the waterbender can't actively see the water that they're bending, it's in their target's body after all. Some might say it's because the water is mixed in with non-bendable matter. However, both of these theories are wrong. We've seen waterbenders easily bend water that is out of their line of sight. We've seen waterbenders effortlessly separate water from sand and soil. 

The real answer for why bloodbending is so hard to master lies within the nature of waterbending itself. Each of the four elements has a certain unique style, a certain quality to it. When it comes to waterbending, the style is very flowing, amorphous, and adaptable. The water becomes a direct extension of the bender's body, the bender's will even.

Naturally, that is the attitude that many waterbenders come to have. They're not just moving water around, they're feeling it through their body, and even their soul. When you think about it, it's no wonder bloodbending is so taboo, especially in Water Tribe culture. It's the exact opposite of waterbending. 

When a waterbender is bloodbending, they're not just moving a ball of water around, they're forcing a human being to be their puppet. They can feel the water in their veins and muscles resisting their will, constricting and cracking in an uncanny and disturbing manner. They're crossing a boundary of a person's body, not to heal as most waterbenders do, but to manipulate, to harm. 

Many waterbenders on a deeply subconscious level reject it, on a base psychological level they can't accept that they can bloodbend. There is a barrier in their mind that resists such a concept outright. It's only when their levels of strength are at a very unusual level, and they either lack conscience and/or are extremely motivated can they overcome this barrier. 

That is why bloodbending is so rare and so difficult. It takes a very certain mindset, a willingness to cross a very intimate boundary, and a certain level of belief in their own power. Otherwise, it simply won't work at all.