What is the wisdom behind Gabriel's appearing to Mary, daughter of 'Imran, as a perfectly proportioned human being?

Gabriel appeared to Mary in the form of a well-proportioned human being so that he could speak to her, and she would feel at ease with him and not be repelled by him. Had he appeared in his angelic form, she would have been repelled. Gabriel was called a spirit because he brings that by which people are enlivened: revelation. His intentional appearance in a good form was a test of her chastity. The manifestation of angels in human form is not strange; rather, it is necessary for their communication with humans, as mentioned in the Quran that if Allah were to send an angel as a messenger, He would make him a man so that he could be addressed.