Platonic love
The thing that surprises Sarah O'Brien most about the Countess of Grantham the first time they meet is how unexpectedly lonely she is. She presents a rather different picture to the world, of course – she wouldn't be a Lady if she didn't – but it takes Sarah all of five minutes to work out just how bloody lonely Cora really is.
She has a husband, a loving one too, but Sarah sees the way he looks at her sometimes, for the crime of having never had the boy she'd been expected to. Her mother-in-law holds the same crime over Cora's head, and she's no bleedin' kitten; even she is reluctant to cross Violet Crawley. Her mother is in America, her children are young, and the only ally the poor cow seems to have is off in London and onto her third marriage, a thoroughly miserable git that her ladyship doesn't seem to hold in high regard at all. It's more than Sarah ever had, by a considerable degree in fact, but she still wants to bash all their heads together for leaving Cora on her own.
So she decides, hardly a month into her new life here at Downton, that she's never going to allow Cora Crawley to be lonely again. And if the loneliness that has defined Sarah's life so far is beginning to fade into memory then it really is nothing but coincidence.