Lyric discussion by smilescart 

Cover art for The Greatest Sum lyrics by Avett Brothers, The

This song isn't sad. In fact it's very romantic and powerful.

In this verse, "This and who I used to be don’t matter much at all to me The pit you dug to plant your feet’s a far cry from my destiny"

He's saying that he's nothing like what he thought he'd be, but he's ok because he's with her and everything has changed. His "destiny" doesn't matter anymore because of her.

Second verse "Dark and lonely is the ride the devil always by my side Though no match for what lies between a thought of you, your trust for me"

He's saying that the devil is all around him, but he's no match for the love he has for her.

Third verse, "No count of gold placed in my hand the largest find, the greatest sum Though some would say it’d easy our pain we know that it can’t be done"

He's saying that the money is said to be the source of happiness. But he says its not and implies that, for him, his girl is what eases his pain.