Lucius didn't know why he picked her he just did. There were many eligible bachelorettes for him to pick from, but Narcissa Black stood out. They had known each other from school and Narcissa was the only girl who didn't fall for his charm. She was the girl he never had and the girl he had always wanted. Narcissa wasn't happy to find out she was begin married to Lucius though.
"The man's an arrogant bastard" she shouted at her mother and then stormed off to her room. When she and Lucius finally sat and talked together she made one thing very clear.
"I will not be your sex toy or baby maker I want to be someone you'll love and cherish forever." It seemed kind of foolish for her to come out with it, but Lucius understood. He had been a "bad boy" sometimes and he regretted his behaviors back in school.
"I can promise you that Narcissa." And even though Narcissa didn't believe it at the time she would learn that Lucius did care about her, and that she was more than just a trophy on the shelf.