This and this were my primary sources for this post, other than my own memory.
1. Sophie loves him.[]
I don't necessarily mean romantically! But Sophie loves our boy so much.
"But he was also the boy who'd shown up on her class field trip and shown her where she really belonged. The one who'd let her cry on his shoulder when she had to leave her family, and who'd gone out searching for her in the middle of nowhere, just because he'd heard her voice in his head." (This quote is from Exile, when Fitz apologizes to Sophie and she remembers why she loves him.)
"Fitz was the first elf she'd met, and the only one who knew what she really gained—and lost—the day he found her."
"I'll always remember that you found me and brought me to the Lost Cities and showed me where I really belong. And that you came when I called for help and saved me from fading away. And that you left everything behind to go with me when I joined the Black Swan."
"What if I want you to wake up because I miss you?" (From Flashback.)
2. He supports her so much.[]
"Sophie had just enough strength left to wrap her mind around the gleaming shards and transmit a call for Fitz’s help. He sent a tidal wave of heat, launching everything up, up, up—through softness and sludginess and pain and relief until she was back in her body, shivering in a pair of warm arms that held her close and careful and wouldn’t let her fall. 'Shhhh,' Fitz whispered. 'You’re back. You’re safe.'"
"If we really need to help him, you’ll figure it out.
How can you be so sure?
She could almost feel the warmth of his smile radiating through the connection between them as he told her, Because you’re Sophie Foster. That’s what you do."
"You're brilliant and talented and beautiful and—are you crying?"
"Sorry," she said as his thumb brushed away the tears she hadn't been able to blink back in time. "Sorry."
"Don't apologize!"
"Why? I'm being ridiculous. It's just . . . no one's ever said anything like that to me before." And to have it coming from him…
He brushed another tear away, and she pretty much melted against his hand. "I'm glad I got to be the first to do it," he whispered, leaning close enough for his breath to tickle her damp cheek. "And I'm going to keep doing it until I make you believe me, okay? I mean it."
"It made me feel unstoppable. And you're a thousand times more talented than I am, Sophie. I know you can do this."
3. He bakes.[]
Need I say more?
4. He fiercely loves his family and friends.[]
That's why he reacts so strongly against betrayal and against things that happen to them. He wants to protect them, but he can't, and he feels powerless. If he's the Golden Boy, he should be able to fix anything, right?
And the way he spends all of this time picking out gifts and baking for Sophie when she's struggling? Ugggg. I melt.
5. He trusts Sophie so much that he wants to be Cognates with her.[]
"I don’t mind having you know what I’m thinking, Sophie. I trust you. Her cheeks turned warm. I trust you, too."
"'Whatever happens next, I'm here with you. You know that right?' Fitz whispered.
'I do.' The best part was she actually believed him."
6. Mr. Snuggles[]
I mean. Seriously. He can't sleep without a sparkly stuffed dragon. How cute is that?
"'By the way,' he murmured, pressing Mr. Snuggles against his heart. 'I missed you too.'"
7. He fights for what he believes in.[]
For starters, he spent six years searching for Sophie, despite it being illegal. He wanted to bring her to his world, the world he trusted and believed was practically perfect.
And no one can deny his passion for fighting the Neverseen or protecting his friends.
8. He cries.[]
I know, a bit of a weird one to put on the list. But call to mind these occasions:
He cried after Sophie's death. He missed her so desperately that, when he heard her voice, he thought he was imagining it to comfort himself.
He didn't yell or break things after Keefe's betrayal like Sophie expected. He just turned away so that no one would see him cry.
He went home and cried after Sophie and him broke up.
9. He's such a dork.[]
The Fitz Short Story mentions that he would lay on his bed with a dopey grin every time something happened between him and Sophie. How cute is that?
10. The breakup with Sophie.[]
I don't think there's a single fan that would deny that Fitz messed up in Legacy. But remember the breakup? They came together, maturely, and agreed to be friends. They weren't angry with each other anymore. They weren't upset. They just decided to be friends, at least for now, while they matured and figured their lives out.
And, during the breakup, he totally rambled about how much he missed Sophie. He regretted pushing her away. And that matters.