I think this look- as I have always thought- depicts Merlin thinking about the future. Take what he had just کہا to Arthur about taking up leadership and becoming king. It's a thought and a very clear thought of the future. Uther is a broken man and Merlin's mind- as he is always concerned about the future- has gone into thinking about that when he tells Arthur this. Arthur responds, "Who knows what the future will bring." Immediately afterwards, the knights come riding in- Gwen leading them on a white horse. Most of these knights are commoners- people who could never have been knighted before. Arthur obviously has gotten up at this point and his helping Guinevere off her horse. They kiss and Merlin watches on. He sees these knights- people whom Arthur would have never knighted before and is reminded how much he and Arthur have grown up. How much their destiny has developed and how he can finally see why his destiny is linked with such a man as this. He sees Gwen and Arthur and sees the Once & Future King & Queen shining through. He sees that this is a sign of the future- that Arthur has in fact already grown up and will be a great king. How he can take Gwen as his Queen. How these knights who have gone through hell- Lancelot particularly- have achieved their dreams. He is seeing a brilliant light ahead of the future and of his destiny that is finally shining through.
So there آپ have it. My personal opinion of Merlin's wistful look :)
I always feel a bit sad for Merlin during this scene, but here is what I think. I believe Merlin has always been the first Arwen fan. (In fact, he is currently presiding president of the worldwide Arwen club.) He's the one who heard that speech that Arthur made in 2x04. He's the one who went through the sorrow, hurt, fear, and hopelessness with one یا both of them in 2x10, 3x06, 3x07, and 3x10. He was with the limping Arthur in 3x12 when he went to Gwen's house to find only Elyan there and had to drag the prince from going to find "Guinevere...my father" in the قلعہ when they got there. Merlin knew better than anybody in the world what these two meant to each other, and seeing them reunited after that horrible experience that they'd all been through, with the kiss right there for all to see, it had to touch him to the core. Actually, his look on the stairs reminds me of the one he had in the coronation scene of Morgana's dream.
And I must add that I think perhaps he also thought of Freya in that moment. Awwww.