When I saw this painting as a thumbnail on google, I thought that a lonely person is standing on a sinking island. But actually a shepherd stands in the middle of his herd.
I do not know if Mauve has deliberately sought this analogy (or if I have made it up), but in principle the emotions in paintings increase when we remind the viewer of something familiar, something he immediately understands, even if it appears in new context.
Suppose Mauve wanted to express how lonely the shepherd is who has to move through the high snow with his herd. Mauve really wants to emphasize and make tangible this emotion of loneliness.
But he can not assume that every viewer knows what it feels like to look after a herd of sheep alone. So Mauve is looking for something that everyone can easily imagine. He thinks and comes to the conclusion that no one is lonelier than a person, on a leaving island, which is only a few square feet tall.
He visually transfers this universal motif to his shepherd and composes the entire painting in such a way that the viewer feels reminded of the lonely man on the island without necessarily noticing it.
So we see the lonely shephard with his herd and we see him lonely on the island.