Mary wasn’t sinless, as Roman Catholicism teaches. Her conception of Christ was miraculous but not immaculate. She was a normal, sinful woman. It would defy grace to suggest that God picked her for the job because of any innate merit, nevertheless, God had to pick someone to host the incarnation. She didn’t volunteer for the job, but her character bears consideration in light of the extraordinary burden she was called to bear, moreover being the only person in history who would ever be called upon to do so.
Listen to this sermon (sound only, no video version available)