Ymir spent her whole life as a slave, mistreated and used like a tool by King Fritz. She was repeatedly abused and used her powers, but never felt the kindness or genuine affection from anyone. She couldn't make decisions for herself. But that all changed when "he" appeared.

Eren: "You're not a slave! Nor are you a god! You're just a human!".

When Eren wrapped his arms around her from behind, she broke down in tears, as if she had finally found someone who understood and cared for her.