Each action in the game has a hate modifier, so certain actions in the game generate more hate points than other actions. What most people do not realizes is every time a monster hits you, it subtracts from your hate points, thus causing you to loose hate.
You notice this the most at the start of a fight. If a white mage casts cure at the start of the fight, and heals a person for 30 points, it will generate 60 hate points. The monster will then come over and hit the white mage. If the monster hits the mage for 120 hit points, the white mage, will loose hate and the monster will return to attacking the next person with the highest amount of hate points, with out the tanks doing anything to draw hate back. This is why flash, blind, paralyze are so important, not just because it reduces the amount of MP needed by the healer, but also to prevent the monster from subtracting hate points from the tank. This is why NIN can hold hate, because the monster never does any damage to them, thus there one provoke and the amount of damage they are doing, keeps enough hate points generated.
This is why I get onto WAR tanks that are using a sword and shield. A shield is only going to take off 2 to 6 points of damage from a hit. Regen alone will nullify that. It is far more important that you dodge their hits, while infecting the highest amount of damage.
This is also why I get onto tanks provoking off me, before it hits me. Let it hit me once, then provoke it. If you don’t then I still have 60 hate points generated and my next cure will just generate even more, causing you to have to provoke again later. This is also why mages should not be healing them selves in battle until it is absolutely necessary. Just cast regen on your self and let it do its job, because it will spread the hate points out over time instead of just one lump sum.