If a tank loses aggro, whose fault is it?

[Llus]

Why do people feel the need to place blame?

Can we not just chock it up to a failed attempt and both try to learn from it?

Why does the tank have to be at fault? Why does the DPS have to be at fault?
If you feel your pulling too much hate and the tank cant keep up (or the healer or dps, or w/e may be the case), than ADJUST for it. Control your DPS, or talk to your tank if pulls are too large that you cant heal fast enough. Whatever the case may be.
Sometimes many factors are at play. Inexperience, gear level, play styles, fatigue, stress, etc... Don't just immediately wag fingers. If you just really do not have the patience for what is going on, leave and get into the next one. It generally doesn't take that long, and is better than everyone at each other's throats and hating the game over such a simple thing.
If its not gonna matter in a day, then why sweat it?

Because that reinforces and legitimizes bad play no matter what class you're playing. We only learn from our failures when other people point them out or we recognize them ourselves (which is much harder to do).