She was desperate. She needed to help her family. Her father had a lot of debt, so when she heard her countries king was looking for a wife, she signed up in hopes of being chosen. Not for herself, but to get her family the money that was needed to survive.
But she never thought she would be picked or even treated as badly as she was on her wedding night. He knows why she applied to marry him, and he now used that against her every chance he can. He will break her down until she loves him.