Having never depended on anyone else but herself, Lillian does not rely on her husband on anything and makes decisions by herself.
She was used to being independent, alone, and free.
As an avid traveler who lived alone for two decades, she was used to going on trips frequently, deciding things on her own, and doing things her way.
She was scared of getting seriously tied down to her husband who might abandon her when he was done playing around.
She had major trust issues in men because she never had a reliable man in her family.
When Leonard realized that he had feelings for Lillian, he wanted to truly settle down with her and have children. But Lillian mistook it for him just wanting an heir and it became an issue since Lillian who had already done her best supporting her family did not want to have children, especially if the said child will become like her to carry burdens that were not supposed to be hers to bear.
