What is one reason why some people don't like the seniority rule?

Generally the people who have not achieved seniority are the ones who are the most against the system. Seniority is a system that makes sense logically, in that those who have achieved the most experience should be the best at whatever is the goal of the organization. In reality, it often times allows those who don't work as hard to gain more pay and responsibility just for achieving nothing more than being there longer than others.