Disclaimer I do not own Merlin.
To the onlookers Merlin always looks cheerful. They see him running around with the knights with a smile permanently etched on his face. He is the one to joke well the others grieve, to put smiles back on everyone's faces. Somehow through all the trials Merlin's been through it seems that he has kept hold of the cheerful innocent soul he's had from the day he first set foot on Camelot. To extent these onlookers would be correct Merlin truly is cheerful in those moments with the knights. His skill of looking forward and not concentrating on the horrors of his past lets him help others through the tough times. Though the smile is not such a permanent feature and one can not simply close their eyes fully to the past.
Fire and magic consuming the lives of Camelot leaving Merlin alone with Arthur till even he is stolen away. Visions of failing Arthur consume Merlin's nights. Then the insecurities these dreams bring hang on through out the days. They are only ever pushed to the back as Merlin laughs with his friends. They are always still suck with him, hiding in the shadows. Merlin's vivid imagination can't help picturing a life of failure,a life without Author. How loneliness would consume him and war would rampage through the lands. He sees the Druids turning their backs to him and friends burning in the fire.
Fire another image that swirls in Merlin's mind. Sometimes he can feel it spreading up his body burning. Arthur always stands over him betrayal written over his face. Pain splits Merlin's heart as he left to burn, there is no room for magic in their friendship. As Merlin lights the fire for Arthur sometimes the heat the yellow flame dancing over his eyes is too much and it causes him to pause. Arthur is always the one to snap him out of it, pulling him away from the flame. His eyes run over the course of Merlin's body making sue nothing is wrong before pushing him towards his next task. For that Merlin is grateful but he still can't help to be afraid.
Though Merlin usually manages to hide from the past sometimes behind closed doors he thinks of what he's lost. Will dying slowly before his eyes, Lancelot stepping into the void after one last goodbye and his father jumping in front of him. Merlin sees these friends, his father sacrificing themselves for him, instead of him. Then there is Freya's pale beauty staining his eyes. All he wants is to feel her arms around him, for her to help him stand strong against what's to come. He visits her lake frequently and talks to the open waters hoping she is listening. At these moments he thinks he feels her breath against his ear blowing sweet words and her cold lips brushing against his. The feeling is gone in an instant though, leaving him alone staring out into the empty water.
So Merlin may still run around the castle's corridors with his truly honest, cheerful grin but this does fade and the innocence he once had has been stolen away. He is haunted by the ones he has lost and fears what the future will do to the friends he has left. Fire burns his eyes when he looks at what magic has done to his home and cold washes through him when he feels alone. Maybe it's only his skill of keeping his past horrors hidden in the shadows that keeps him here alive. It lets him be happy when he's with his friends but maybe one day it might not be enough.