So, okay, I've got to know this..
Why do some dps attack whatever willy-nilly target they like? If you're so geared that you can out-hate me on my main target, put me to shame, I welcome it.
(No really, because if you're that geared, letting you go all out lets the run go so much faster).
However, most dps are not that geared, so let me explain how this should work.
I'm the tank, I pick (and I always mark my main target!) a target because I'm the first into battle. I've also got to deal with the brunt of the damage, and dodging, so this is a privelege of being a tank.
If you attack something else, the healer has to heal you, and cause more threat on mobs than if they were healing me (simply because you require more healing than me). I could fight you for hate, but then, as usually happens, the other dps is hitting my marked target, and he may catch up to my enmity if shenanigans start early enough in the pull.
As long as the ordering is right, the quicker each thing dies, the easier the pull gradually becomes. For instance, if there's a pack with a healer mob in it, that should probably die first (always has so far). The quicker that dies, the less healing other mobs receive, and the shorter the fight goes. As the fight goes on, with mobs dying, healers have to heal me less and thus produce less enmity. This may let me slow down on my aoe enmity, recovering some mp (or tp in warrior's case), for the next pull.
If you're attacking a target different than mine, you are Doing It Wrong.
There is an exception, I will switch target when I have more enmity than the mob has health remaining (ensuring that it will hit me til you hit it to death). I do this because it lets me pop up another marker, lets me use flash a bit less, and let's me put my str down debuff on something else (I can do that with sub-targeting, and do, but this still helps).
Technically, any time you pull hate off a tank, when he's playing properly, you're doing it wrong, but like I said. For me, personally, if you can crank so much DPS that you outhate me, I welcome it. Get that trash dead. But please, give me a break on bosses.
However, some tanks don't take to it so kindly. It can be frustrating to deal with and this is what causes some tanks to enter the instance on the offensive. For you behaving dps, and healers, note that for every rude tank you've encountered, he's probably encounter 5 dps that just do whatever they want.
I recall, humorously, the first pull of one AK. I targeted the summoning caster, started beating it down, and a drg pulls hate on whatever he's attacking. The other 3, all friends, pipe up "Why was Polly Arm tanking?" (Polly Arm is a made up name). I reply "First, I mark my target, he should be attacking that, and second, on the pulls of these casters, it's important to get the one with the staff dead first, before he summons a demon." Some whining ensued, some d-baggery, and I politely made my exit and let them wait for another tank to troll (sorry to whoever that unfortunate fellow may have been).
Please though, until there is better reason to attack a different target than "It's hard to tab", consider that we're there to do a task and you can make it so much easier by just following our order.
Haha, yeah, but see that's okay, and that's how the job is meant to be played. I'm talking about people that specifically focus on the wrong target.
Mostly I get tired of dps in full AF feeling like they can tank. Sometimes I let them die, but usually not, because even their mediocre dps speeds the kill up.