Curate, connect, and discover
Just occurred to me that from Arthur's perspective yes Merlin seems really brave and stupidly loyal but also like someone with a death wish. Not necessarily wanting to die in the straight forward sense but reckless with his wellbeing in a way similar to characters like Lockwood from Lockwood & Co. Arthur doesn't know Merlin has magic so he has no reason to think Merlin is anything other than a defenceless servant who is reckless with his life to a concerning amount.
I've seen discussion and fan fics exploring how it should have been obvious to Arthur that Merlin was in love with him because of how devoted to him he was in such a self sacrificing way but I think you can justify Arthur not assuming Merlin's behaviour was born out of love for him because from Arthur's perspective it seems far more likely that Merlin just doesn't value his own safety and life to a concerning level. He's always joking that Merlin has a mental affliction, doesn't question truly strange behaviour because oh that's just Merlin being Merlin he's an unstable weirdo. Arthur counting the days since he saw Merlin smile because gods forbid Merlin's reckless lack of care for his own life developed into something with more intent.
Basically I think Arthur believes Merlin is legit just mentally ill in some way. Because frankly without the contact of Merlin's magic and the prophecy his behaviour is really concerning (frankly even with context it's pretty concerning by the final few seasons, that boy is not okay) and because to Arthur that is a much more logical explanation for Merlin's behaviour than Merlin actually loving him (platonically or romantically, doesn't matter) because Arthur can't comprehend someone loving him like that.
Also makes his reaction to the magic reveal make some painful sense because he's never factored Merlin's love for him into his understanding of Merlin's behaviour so when the weird behaviour he'd assumed was a mental affliction seems to be revealed as just a lie to hide his magic Arthur has no basis of Merlin's devotion to him to fall back on and use to make sense of why Merlin would be loyal to him despite having magic.
Arthur doesn't see Merlin's self sacrifice as the result of love from him but as a result of lack of love for himself.