Merlin and Arthur are entwined by Destiny, like two hands wrapping their fingers together. They don't love each other.
They are together in all respects, but it is so unrespectful towards the world, and towards Destiny and towards what is goddamn appropriate that they aren't really together at all. Because, really, the world has never shown any respect to them, so why bother?
But Arthur would always be fascinated by Merlin, because Merlin is just so different. A young man with pale skin, dark hair, blue eyes, and a personality that is a mystery wrapped in an enigma - Merlin is an adventure, and Arthur could never resist an adventure. But he would never say he loves Merlin.
Merlin would always be entranced by Arthur - by the way his hair glinted in the sun, by the way his angelic blue eyes, by the way he fought for what he thought was right every time. But he would never say he loves Arthur.
'I Love You' would never be enough.
The way they feel could never be encompassed by a word, a definition - it is so much larger than that, millions of times larger. The way they feel is something that floats in the sky, the heavens, through time and space, and it could not ever be captured and tied down to the ground by a word, 'love'. No bard, no poet, no scribe, could ever write down exactly what it is, never. Neither could Merlin or Arthur.
Because they would never say 'I Love You' - it isn't what it is, how they are. Arthur will never kiss Merlin. Merlin will never hold hands with Arthur. They will never fall in love, because what they have isn't love. 'Love' is no more than a shadow compared to the things passed between the lingering glances.
Arthur and Merlin never realize it, not ever, but it is always there. They will be bound to each other through Destiny, and then through friendship, and then through something that is so deep its echoes reverberate throughout the stars, the heavens, the universe. It will never leave them, even as time passes beyond the two.
And they will spend their lives in something deeper than love, being together but not together, and the things they do together are recorded in the history books, but books have words and words do not have the things Merlin and Arthur have together.
Even if it is skinny (not skinny 'love', mind you), they are forever. From the way they look at each other with feelings in their eyes, saying things they will never say, from the way they defend each other to the ends of the earth, from the way they need each other.
Even at the last moment, they never say 'I Love You', because it doesn't encompass the way their forever lasts throughout all of the universe.
It doesn't need to be said, anyway.