It's because when he smiles, it lights up his face, and it's always genuine.
It's because he talks about sandwiches for hours and has a Tron poster he got in high school; it's because he can be willing to go undercover in a situation that has about a 98% chance of getting him killed, because he knows it will help people. It's because he's totally innocent in a way that has nothing to do with sexuality. It's because he believes in the good in the world, and because he remembers what amounts to three separate Intersects and isn't brain-dead, can still reconfigure a computer in just a couple minutes. It's because everyone at the Buy More looks up to him, even Big Mike himself, and because Morgan is ridiculously attached to him.
It's because your heart beats faster around him, and because even though you never give it away, you want to apologize for how you act. It's because you tend to unconsciously try to show off around him, especially when he's in a relationship. It's because you can relax around him in a way that you forbade yourself from ever doing a long time ago. It's because when you touch him, heat sparks in your fingers and travels up your arm, and he doesn't even notice. It's because he talks in his sleep about things like his life as Agent Carmichael, and lobsters, and how to build a computer. It's because when he's drunk he falls into you, all hot and sweaty and goddamned sincere smiles and curly hair and fingers sparking on your shoulders, and grins and mutters things that you don't quite hear because you're too busy focusing on his voice, the beat and pulse of it matching the music, rather than his words, and because it takes far more alcohol than anyone would expect for him to get drunk at all. It's because you trust him, probably more than you've ever trusted anyone, and you think that he feels the same way about you, even if he doesn't feel the same way. It's because sometimes at night you just want to forget and kiss him but you have too much self-control and you don't think he'd want it, anyway.
It's because he speaks Klingon and never forgave you for Jill and Stanford – it's because he doesn't trust you anymore and when he looks at you, the old admiration is mixed in with a new, quietly sad acceptance – it's because you know you couldn't have killed him and he just knows that you were about to.
It's because, despite that, Chuck still will smile when he sees you, and it lights up his face, and it's always genuine.
In case you didn't get it, the second to last paragraph is Bryce - Sarah - Casey.