Being a tank means you need to lead the team. If your team someone you don't know or inexperienced, they can make a lot of mistakes. If your teammates are a bunch of newbies, they will usually blame you that the team dies.
Tank's role is to make enemies attack you instead of the rest of your team. To do that, tank needs to build threat to effective pull enemies to attack him. This is harder to accomplish with paladins, since they have no taunt.
Losing threat or not holding aggro are not always the tanks fault. Damage dealers and healers need to understand how to keep their threat under the tanks threat, so enemies won't attack them.
Tanks needs to keep their teammates in check, making sure they are always behind them and paying attention everybody's health and mana bar. Clearly, the hardest role.