HIII long response but I did write something for this originally in one of those shipping qnas a bit ago so I'll post it again. I'll probably write dislikes/nitpicks right now but usually it takes me a little bit to finish my essays like it did for this one.

[Physical]
Gay as hell but Itsuki really likes traits in Sona that he'd find in Madoka. It's never vocalized but he really likes his eyes. Him and Madoka have the same shade but somehow, Sona's visage and expressions are a lot softer than Madoka's though so at the same time, it's like he's seeing a whole new different perspective. (This is unhealthy thinking on his part.)

Itsuki also likes his physique, which also reminds him of Madoka, but it's not in a worshipping way like he used to. With Madoka's body, despite them being in a relationship, it was something to admire, she was as elegant, ethereal and powerful. There was not much else to it. Like one aspires for the heavens, they don't actually believe it's tangible for them right now. Maybe one day when they finally deserve it.

Sona isn't like that though. Sona isn't a god. Sona isn't ethereal, and he's certainly not elegant. Sona was awkward and weird, but at the same time, he didn't feel completely human so it wasn't completely new territory. Things were different, for starters, he's allowed to touch Sona without permission. It's an action he has to settle into but Sona encourages him to hold his hand, to cradle his face and to run his hands up his body. It feels like taboo though to indulge in love so he doesn't do it for a long time . Everytime they do anything affectionate, there's a voice in his head that there's something wrong with this, and him, so he shows restraint and one day he will die and everything in the world will maybe click.

Sona notices Itsuki gets uneasy with touch so he doesn't set initiative.

It's not like they aren't physical though, it's just that Itsuki is more likely to smack Sona's hand away than hold his hand at first.

Tldr; Itsuki's favorite parts of Sona are whatever reminds him of Madoka.

Sona thinks Itsuki's really pretty. He didn't fall in love at first time but was enamored by Itsuki's appearance when meeting him. Every part of Itsuki is always so collected and put together that he's lovely to watch from afar like a movie he has no part in. Sona wouldn't ever say he has a favorite part of Itsuki, that feels really shallow but he likely does have one that he feels ashamed to praise about.

I'll probably say his neck for this one? I am the creator of all worlds and people here so I can make up whatever and have it be canon. Itsuki always has it obscured by his scarf so I think he really likes ogling at it from time to time when he gets the chance because it doesn't feel like Itsuki without it. It's likely a rarity to be able to touch it so he likes kissing his neck when he gets the chance. (This one has always been canon, although this was more of an nsfw fact because Sona likes to give Itsuki hickeys. I don't talk about it though cause it seems kind of cringe.)

Personality:

Itsuki doesn't Know it yet and wouldn't be able to articulate it, but Itsuki admires Sona's own independence and agency. It's something he lacks and he finds the prospect of being your own person very terrifying. That's why when Sona staggers in his own choices or freedom, he kind of gives him this admonitory "I thought you were better than that" spiel that does help Sona occasionally. He doesn't try to pry into Sona's life and respects him for who he is currently.

Sona admiring Itsuki's personality is a big part of their relationship and his feelings. Sona believes with his whole heart that Itsuki is a good person. Itsuki has that humanity he lacks. Itsuki is passionate about everything he does and is authentic unlike him. Sona will time to time admit he admires Itsuki or calls him a good person and it baffles Itsuki because he /isn't/ his job involves illegal activities, he's watched people die, he's disposed of their corpses and every interaction he has leads to discord and chaos. It's very patronizing for him to hear this, but eventually, he wears down and tries to talk Sona down from his weird delusion of his morality.

The love Itsuki has is something he really adores as well. Itsuki dedicates himself to the people he loves so well. Sona never had a relationship where he was deeply in love so Itsuki talking about Madoka and what he does for her is really cute. He just wished their relationship wasn't so sinister. To love so passionately and selflessly though, that's not for someone like him but perfect for Itsuki.

And last of all, all of these tie into Itsuki's humanity. Sona has low empathy and was referred to as nothing more than a cog as a child. With this, he's convinced himself that he's not really a good person, much less a person. He's unable to grieve and love like the typical person, he has to fake his way through it and even his need to be a good person isn't genuine so all he can do is try his very best. Itsuki doesn't need to fake any of this though, which is amazing for him. Every thought Itsuki has is not thought through and comes from the heart and when Itsuki cares, he does care, deeply and Itsuki isn't afraid to tell people what he feels either. Honesty is scary for Sona and is forbidden in a way. He doesn't do it on purpose but he lies more often than not to hide his own feelings or to make people like him.

Itsuki also insists that he doesn't grieve, but he knows that Itsuki does feel guilty over the people his friends killed and corpses he's dealt with which is something he really likes even if Itsuki himself doesn't enjoy it.

On the light hearted side, whenever Itsuki tells Sona he loves him he knows he means it earnestly and it flusters Sona because there's no reason for Itsuki to lie/Itsuki doesn't amuse formalities so he'll be laying down in bed and be like "wow......my boyfriend loves me......."

Anyways Sona is in love for how "human" Itsuki is and Itsuki is in love with Sona for nostalgia/ethereal reasons and Sona's own individuality.