One argument is that, traditionally, wives had to be supportive of their husbands' careers and aspirations. By pushing Macbeth to be king, Lady Macbeth could be fulfilling her role as the devoted, helpful wife. It's also a demonstration of her love for Macbeth, as she wants him to succeed. Alternatively, her motivation is her own ambitions and goals. Her position as a woman in society means her own career prospects are limited, so the only way to move up the social ladder is for Macbeth to. She wants to be queen and reap the benefits of Macbeth's successes. Furthermore, becoming queen could be compensation for her lost children. If we do conclude that she had children but they died in infancy, then Lady Macbeth has lost her 'purpose' as a woman.