When anyone else protects her, she feels inadequate. She thinks of herself as a burden. She loves everyone (Tatsuki-chan, especially) who comes to her defense, but she always ends up hating herself a little. She doesn't say anything, only wonders why they all love her still, even when she is a burden.
Orihime knows it's silly, and berates herself for crying about it. She should worry about more important things, like Kurosaki-kun or the Arrancar, instead of concentrating on her own selfish wishes.
But when it's Sado-kun, it doesn't make her feel quite so useless. She lets him protect her, and doesn't object - neither consciously nor unconsciously. She doesn't feel weak, because she knows that he's got something to prove, too. Sado-kun wants to protect everyone, whether or not they need it.
Orihime can understand that, because she's never been able to protect anyone. She admires him, because he does everything in his power to protect everything precious - not just his friends, but the ants on the ground, too.
It took a while for Orihime to discover that it was somehow different with Sado-kun. She couldn't put it into words, but she felt differently about him than anyone she'd ever known. And that, she thought, was something to hold onto.