Arthur has a really, reeeaaaally bad singing voice and no musical talent whatsoever. Merlin on the other hand can actually sing very well and is often found humming one tune or the other.
So of course Arthur gets annoyed any time that Merlin starts singing softly under his breath while he is doing his chores. Because how can such an idiot (Arthur’s words, not mine) have such a beautiful, melodic, perfect voice?!
Of course Arthur’s jealousy subsides once Merlin starts actually singing for him. When they’re out hunting and setting up camp and Merlin starts humming some irritating tune it is no longer annoying but rather endearing.
And in the early hours of the morning, when the only ones awake besides them are the cooks in the palace kitchens and the bakers in the lower towns, when Merlin and his Prince are lying side by side in Arthur’s warm, cozy, comfortable bed and Merlin sings softly, Arthur doesn’t have it in him to feel jealousy. He only feels love.