If you don't say Hello at the beginning of the dungeon you're off the list by default.

Usually I give my com to a healer or tank but after reading this thread I'll include Dps more often.

If I tank and I see people letting me go in first to grab mobs -> commendation worthy, I appreciate the courtesy.
If I'm a healer and I place my bubble of protection down and the tank or dps actively step out of it for some reason - > no com for you.
If I'm a Meele dps paired with a ranged and they do not come close for buffs (brotherhood, dragoon tether) - > you'll not get the com.

If you're talkative and overall pleasant to be around I'll always give you my com.