A kohen’s duty – Naso
The original plan was that the first-born sons would be the spiritual leaders.
Then the kohanim were appointed in their place, which is why the kohanim and not the first-borns bless the congregation with the Priestly Blessing that is sourced in today’s Torah reading.
Why were the first-borns replaced? Tradition attributes the change to the time of the Golden Calf. The sin involved the first-born as well as many other members of the people.
The kohanim, however, took a firm stand against any form of idol-worship. God therefore decided that the kohanim warranted special status and privileges.
Some modern kohanim are overtaken by a feeling of modesty and try to shrug off their priestly duty. Strangely, they have no problems with being called to the Torah at the head of the list; other things which they regard as onerous are a different matter.
The Torah is quite clear: being a Kohen, with all its privileges and responsibilities, is a sign of Divine confidence and every kohen should wear his kohanic distinction with happiness and pride.