Merlin hate's sleeping with Arthur. That is to say, he hate's sleeping next to Arthur.
Arthur is a deep sleeper who often talks's nonsense as he slumbers. Loud nonsense that keeps's Merlin awake. And that's without all the tossing and turning.
And if he wasn't talking in his sleep, he is as still and silent as the grave. Merlin hates that, and always manages to convince himself that Arthur has died.
Arthur is never pleased when Merlin wakes him in the night to make sure he's still alive.
So it's a pity really, that Merlin so enjoys sharing Arthur's bed.