Yuuri > Nico
It's like looking into a mirror. That upbeat personality, the hero's spirit, the occasionally silly... Oh, wait, he was looking into a mirror. This is about Nico. Well, they do say opposites attract.
Yuuri cares a lot about Nico. I mean, he is the kind of person who would want to save everyone all the time, so there's that, but he manages to think of Nico more than when the boy is right in front of him. And he's been grateful to have a companion in the prison. He's had so many people disappear on him and even before disappearances there were problems.
Once upon a time in the prison he met a boy named Joshua. There was a kiss. This spiraled into (IC) months of Wolfram hating Yuuri and distancing himself which was a loss that affected Yuuri greatly. It was only shortly after making strides toward reconciliation that Wolfram got lost in the stabilizer. And then there was Conrad who had always been there for him. Who was one of Yuuri's most important people. He was turned into a chiropteran and his allegiance switched to an evil crazy lady. While he still cared for Yuuri, to protect him he kept his distance from him once again leaving the demon king alone. Eventually, Conrad was free of her hold and he and Yuuri were closer than ever and then can you guess what happened? A million points if you said "lost in the stabilizer."
He also lost Joshua and the first fledgling reciprocal romance he'd had. And then there was his mom, other people from the demon kingdom, and a great many of the friends, mentors, and people he'd decided to protect. Not to mention everyone who is still at home(s) and always has been who he hasn't seen in nearly an (IC) year. He's been in the dome since Day 10. That's the longest of anyone, so in some ways he's lost the most.
Yuuri seems to take this in stride and for a large part he does though it does occasionally catch up to him. It's somewhere in his low points where he can relate better to Nico's darkness. And it's in his high points that he wants to make the other boy's life better than it has been.
There's also simple math. Two boys living close together, getting knocked around by the wardens, bodies pumping with hormones and all full of time and opportunities. It's not shocking that kissing happened. Yuuri isn't always sure what to do with this boyfriend thing. Joshua tended to take the lead before and Nico isn't anything like Joshua. But he doesn't mind taking it slow and just having Nico's company and he's slowly coming around to this being his normal now and not freaking out over it.