Tanks don't naturally build increased emnity without tank stance or emnity abilities. At lower levels DPS generally don't do enough to threaten taking aggro as long as you use one or two aggro abilities per pull/every so often during a boss fight, the initial ones can generate a lot, like a tank stance'd butcher's block from a warrior can hold hate for a while if you just plain dps after it.

Gets riskier the higher you get though and at 60, mobs live longer, DPS gets higher, you'll generally have to use some form of emnity gain whether emnity abilities or just tank stance