Due to his childhood (full of bullies, specially from his family) Arthur really, really, REALLY has lots of self image issues. 99% of the time he can’t see himself as beautiful or handsome. Only as ugly. Most of the world finds him handsome, but he can’t see that.
When he is really sad he likes to try to look into the mirror and pretends that he is pretty. Usually that ends up with him crying on the floor, feeling undesirable and miserable.
That was one of the reasons he refused to be in a realationship with Francis for so long. Even when they are together he is extremely insecure (because what if Francis run away with somebody more pretty?).
When Francis found that, he was horrified because how dare anyone to feel ugly in his presence! So he makes his mission to make Arthur feel beautiful and loved. Little things, like write in the mirror “hey handsome ;D” before Arthur wakes up, worship his body in small ways (like kissing his hands and saying they are perfect, prase his eyes, run his fingers through his hair, etc), never missing an oportunity to say that Arthur look dashing, and etc.
That really helps Arthur