When Merlin saw Arthur for the first time, he could not deny the man was attractive. It was his prattish behavior that kept him from falling in love.
That didn't last long, though.
The problem was that Arthur became so much more than the clotpole that he once was. In secret moments, Arthur had kind eyes and gentle words. He revealed the side that he kept hidden from his cold, distant father and arrogant knights. Arthur became a true king; the king his father never could be.
Arthur was bright -blindingly bright- and Merlin loved him for it. Arthur was a distant, untouchable star that brought light into his formerly dark world.
I'm building this house, on the moon
Like a lost, astronaut
Lookin at you, like a star
From a place, the world forgot
And there's nothing, that I can do
Except bury my love for you
Merlin knew what he had to do, but that didn't make it hurt any less. He and Arthur could never be together, not matter how deep his love was. So, he stifled his love. When he felt the overwhelming need for Arthur, he stuffed it down, ignored the need to hold him and kiss him and love him.
The brightness of the sun, will give me just enough
To bury my love, in the Moondust
I long to hear your voice, but still I make the choice
To bury my love, in the moondust
I'm a cast away, and men reap what they sow
And I say what I know, to be true
Yeah I'm living far away, on the face of the moon
I've buried my love to give the world to you
Even when Merlin saw the unbridled adoration in Arthur's eyes, sometimes—dare he hope—desire, he buried his love. He had no place in Arthur's life as a servant, a man, and a sorcerer. So, to give Arthur the world, he denied himself.