Author's Note: And here's Bellamy's end of things.
She never says the words. But, if there is anything he's learned about Clarke Griffin since he's known her, it's that there is a reason for everything she does. And she wouldn't spend every night ensconced in his arms if she didn't want to be there. Bellamy doesn't blame her; he can figure out why the words never cross her lips for himself.
It doesn't take a genius to figure it out, especially when he's woken up by her nightmares most nights. (He's got nightmares of his own, so he's not complaining about being awake instead of sleeping.) Sometimes she screams for her father, sometimes for Wells: time may have passed, but those are wounds that can never truly heal. Sometimes she pleads for Lexa to take her instead, sometimes for Finn to forgive her.
Before, he might have been bothered that someone else had the power to occupy the dreams of the woman he loves – the old, whatever the hell we want Bellamy would have hated it. But the man he is now realizes that there's no victory in being the dead man, and dead is all that Finn is now.
And all of these people – her father, Wells, Finn – are why she never says the words. They're so simple, so small, but to Clarke they're like a death sentence. It's hard not to look at them like that when most of the people you've said them to are dead. Bellamy knows how that feels, to an extent. He's only ever said the words to two people, and his mother suffered the same fate as Clarke's father while Octavia might as well have been dead for all the life she had.
But he knows love is what he feels for her. There's no other word for the way that his heart leaps when she presses closer to him while they're sleeping in order to share some of his body heat, or the way he stares, slack-jawed, at her in the evening light with the setting sun shining through her hair. He loves her.
But he can't say the words either, knows doing so would only make her feel guilty and she's had more than enough of that for a lifetime. Nevertheless, he still tries to show her that she's special. I need you, I want you, you're mine, Princess. He doesn't say love, but damn if he doesn't pray for Clarke Griffin to use that amazing mind of hers to figure it out.
By the way she smiles at him every morning, or the way she can totally relax when they're alone together, he's guessing she gets it.