Merlin had lost count of how many years he had lived without Arthur. When the number hit 1,500, he was too depressed to keep counting.
Arthur had been his everything. He lived his life for Arthur; not totally healthy in retrospect, but he was too young and in love to care.
Arthur felt the same way about him. When Merlin confessed about his magical abilities, Arthur didn't treat him any differently. His magic was all for Arthur anyway.
After Morgana's final siege, his surviving friends had died one by one: Gaius, Percival, Gwen, and Leon weren't bound to the earth like he was. And how could Merlin ever love another? He and Arthur were twin souls. He could never have that with anyone else. Wouldn't, couldn't, shouldn't have that with anyone else. For Merlin, there was only Arthur, and he would wait as long as it took.