He's not there for Arthur, nor is he there for the sake of Camelot, or even for the future happiness and safety of the surrounding lands.
It's when Merlin lays his head and sleep captures his mind, Gwaine knows why he's here.
Arthur doesn't appreciate Merlin. Gwaine knows this, but there's something about Merlin, something special, something that he senses and knows to be important.
Gwaine's eyes fix on Merlin's young face, he wonders how different his life would have been if he had someone like Merlin in it. He wonders if he and Merlin ever could have shared the bond that Arthur and Merlin seemed to share.
The light from the fire dances across Merlin's sleeping face and every fiber of Gwaine's being goes to prevent him from touching the sleeping boy. He forces himself to lay opposite the fire and close his own eyes, longing and jealousy his companions on the cold night.
He drifts to the warm world of sleep, only to be disturbed when his companion mutters.
"Arthur," Merlin moans in his sleep. "Arthur…."
Gwaine closes his eyes, that's why he's here. He needs Merlin to be happy, and Gwaine knows that Arthur is the only one who's capable of that.
That's why he's here, for Merlin.