Acceptable reasons for losing aggro:

1) The tank forgot to flip to Shield Oath once inside a dungeon (That's happened to me on occasion. About as embarrassing as coming in with Cleric Stance active, really)

2) The tank couldn't get to every enemy to lock in aggro (Flash helps, but for only so long against sustained damage).

Every tank should hate losing aggro but be equally determined to get it back if lost.