What makes the song for me is this verse (verse 17 of Child's version A):

'Is not thy hawke upon a perch?
Thy steed eats oats and hay;
And thou a fair lady in thine armes,
And wouldst thou bee away?'

She's right - in that moment everything's right with the world for Musgrave, he couldn't be in a better place. But at the same time she's horribly wrong, and he's in the worst place possible (for both of them). That contradiction between contentedly being in the moment & anxious foresight - and between doing what you want & thinking about what's going to keep you alive - really resonates with me.