When you tank, you can see more clearly of what your teammates are doing. When the SMN refuses to Bane, when the BLM refuses to AOE, and when the DRG refuses to move away from AOEs because rotation. You can see why the mobs don't die fast enough or when your HP don't fill up fast enough.

Then you call out to them what they are doing and suddenly you're the bad guy.

"BLM, I think we need some AOE instead of single target. They'll die faster."
"You're not my boss and I do what I want!"
*proceed to early pull the rest of the dungeon and die when healer can't heal him fast enough

That's why nowadays, when I see somebody messing things up, I'll stay quiet and rush things to finish it. I stopped caring.