I'll DPS when i know the Tank is going to take minimal damage, such as on bosses or small Trash packs.

What drives me absolutely insane is the people who demand that i should be DPSing at ALL TIMES.
Yeah sure buddy, let me just stop healing the Tank who pulled 20 mobs onto himself so i can lay down some awesome 25 Potency AoEs.

TL;DR, People are idiots.