Neitherse
Lancelot loved Gwen. He had loved her since they first met, and thought of her every day. He had lived for her and fought for her, and became the man she believed he would be just because her faith in him kept him going.
But now, it was too late, for Gwen had fallen in love with someone else.
Gwen loved Arthur. She had grown to love him slowly, as he grew to be a man, a worthy prince, a future King. She loved him and believed in him until the very last moment. She knew he would be worthy of his crown, and that Camelot would become a better place, and that he would bring an era of happiness, peace and prosperity.
And while Arthur acted as if he loved her, she knew his heart really belonged somewhere else.
Arthur loved Gwen. He always said that, hoping that it would make it true. He couldn't, wouldn't admit that he loved someone else, someone he could never confess to love, that he could never have by his side. He couldn't say that his heart beated faster when he heard Merlin walking, or that his slept in sheets in which his servant's smell clinged. He couldn't touch him, couldn't do anything, he had to keep it to himself, and if sometimes he looked lost in Merlin, he pretended that his head had been far away all that time.
But it wasn't true, and Arthur could never burden Merlin with that knowledge, knowing that he didn't feel the same way.
Merlin loved Lancelot. He loved the knight for accepting him for who he was, for keeping his secret. He loved Lancelot since the very beginning, for they shared the same beliefs and walked the same path. And if Lancelot loved Gwen, Merlin could deal with that, for Gwen was lovely, and Lancelot was the best man he had ever known and deserved to be happy in any way he chose to. It wouldn't change how Merlin felt about him, for he was used to love and not being loved back, Lancelot wasn't the first - the only - one.
None could ever say what truly lay in their hearts, and they act out of duty all the time. Merlin allowed arms to be around him, and Gwen pretended she believed in pretty speeches, and Lancelot act as if stolen glances were enough, and Arthur was a Prince and a King, that could never show any form of feeling for someone so inappropriate.
They were not happy, but they could play their parts well - they were dutiful, honorable and loyal, and would never aspire to have more than what they were supposed to.
And that was the doom of them all.
(Had they said it, just once, Gwen would be a happy knight's wife, Lancelot would show Merlin that he wasn't the only one that deserved that love, Merlin would discover that even his craziest dreams could become true and Arthur would have everything he always ached for.)