This was a fun read describing hate in general and pets:
http://www.magecompendium.com/pet-behavior-library.html
NPCs maintain a ranked Hate List of immediate enemies, and, in general, will chase down and attack whichever enemy within Attack Range is currently the highest on their list. The Hate List is constantly changing as characters against the NPC and its allies. The following discussion is simplified somewhat to focus on pets and their behaviour, but applies, with some modification and caveats, to NPCs in general.
Hate: Direct and indirect actions against an NPC add a specific amount of Hate to the score of that individual on the NPC's Hate List. The value of the Hate added appears to decay over time, so that more recent actions weigh more heavily than those a while ago. That is, Hate appears to "age," and become weaker over time. That is why one must be careful how often one nukes an enemy.
The primary determinant of the Hate Value of an attack is the potential damage that attack could have normally done. Thus Hate ignores both resists and criticals. All attacks with the same weapon or spell generate the same amount of Hate regardless of how much damage they actually dealt, even if they miss or are fully resisted. Spells that do not do damage have a Hate value assigned to them. Some, like Tash and Slow spells, have a high Hate values. Others, like Cancel Magic or Malosini, have a low Hate value. Healing and buffing an NPC's enemies also generates Hate.
Taunting a second time: When a character successfully Taunts an NPC, that character moves up to the top of that NPC's Hate List, but just barely. By default, pets have Taunt On, which means they constantly try to taunt their enemies. However, because of special rules designed primarily to prevent abuses of Charmed Pets, an NPC will almost always chose to attack a Player Character if one is within melee distance, even when a pet is at the top of its Hate List. It is not known precisely what these "special rules" are, but in practice they prevent pets from tanking in the presence of player characters within melee distance of the mob.
Any creature that attacks a pet's owner is added to the pet's Hate List.
Pets cannot attack a creature which has been charmed by a player character, but such creatures remain (?) on the pet's Hate List.
If a Mezzed enemy reaches the top of a pet's Hate List, the pet removes (?) from its Hate List instead of attacking.