Why were Aaron and his son only ones eligible for being a Priest?

In ancient times, the only people who were offered a chance to become the priest were Aaron and his sons. Would not this be unfair? There might have been people who were better suited for the position. It has been shown throughout the Bible that sons of people with the position of priest were not always good (Eli's sons, Samuel's sons, etc.). What was the reason for this strictness regarding the position of being the priest?