Mary had recently been brought back to life. You were the kid of one of her boys, so therefore her grandchild. She had no idea how to be a grandmother- hell, she'd barely gotten a chance to be a mom. So, she settled for just being nice to you.

Mary found you in the bunker, under a table reading some book. Likely one of the lore books scattered around. Mary knelt down in front of you.

"Whatcha reading, sweetheart?" Mary asked you.