Merlin is terrible at the things that most servants are good, or at the very least, adequate at. Arthur's lost count of the number of times he's fallen asleep when he should be working, spilled wine all over Arthur's clean white shirt, dropped his armour, he could go on and on with all the examples he has of Merlin's ineptitude.

But this. This is what Merlin is perfect at. Gentle fingers and wet, perfect mouth. Making him forget about all the things he's done wrong, making Arthur forget that he was ever annoyed or angry.

When Merlin touches him, Arthur has a hard time thinking that he is anything less than the perfect manservant. Has a hard time thinking anything at all except "Yes" and "Right there" and "Don't stop, Merlin, please don't ever stop."