Why were first-born sons supposed to serve G-d?
On the one hand, we could understand that first-born sons are valued, just like first-born animals, because G-d wants us to recognize that our creativity is all from Hashem.
But perhaps there are other dimensions as well? After all, older children are supposed to help look after younger children, to invest in their siblings. Might that be another reason why first-born children would be sought for divine service?
We know that the Levites “earned” the right of divine service after the Egel, the Golden Calf. They showed better judgement than did the first-borns.
Might this be connected to the namesake Levi, who was named in hopes that he would facilitate the relationship between Leah and Jacob? Might this also explain why Levi had the fewest children in Egypt? We only have so much we can do, after all … so the more we invest in others, the less we are able to invest in ourselves. By this logic, if the Levites helped other peoples’ families, then they had less ability to help their own!
Might G-d also switch from the first-born to the Levites because the first-borns in the Torah never seemed to achieve their potential? Think of Ishmael, Esau, Reuven…?