Mary may be goofy and gleeful, but she's not as outgoing as her older brother. It takes her a while--sometimes a very long while--to warm up to people. But Mary took to Aunt Rachel like a fish to water.
I think it all started when Rachel fitted Mary with a fashionable turkey crown. Mary likes shoes, socks, hats, jackets, and all things accessory. So giving Mary a headpiece of her own was bound to win her over.