because she was mine
"I lost someone special to me, too," Lexa tells her by the embers of Finn's pyre.
This is startling. This is... sympathy, or something like it. As close as the Commander can come, at least. Clarke watches the Commander sidelong, wondering where Lexa's going with this: in their handful of conversations, the Commander's never yet said anything simply for the sake of hearing herself talk.
"Her name was Costia." Lexa's voice is conversational, matter-of-fact, but her knuckles are white where they are folded over the hilt of her sword. "She was captured by the Ice Nation, whose queen believed she knew my secrets." Only the whiteness of her knuckles and the faint tension in her jaw, the momentary twitch of a muscle in her neck, betrays that she feels anything. "Because she was mine, they tortured her. Killed her. Cut off her head."
Clarke cannot believe her calm. Cannot believe her when she says -- resigned, accepting, as though it were the most natural thing in the world to have simply stopped caring, to be able to turn your feelings off at will: Love is weakness.
Later, too much later, she will understand what Lexa was trying to tell her. Lexa does care. Lexa cares deeply and intensely, but Lexa holds in her care the lives of not just dozens or hundreds of people, but thousands. (Maybe tens of thousands: the Ground is wider and far more populous than Clarke could have ever imagined, on the Ark.)
Lexa will not put her own wants before her people's needs, and love is selfish.