Not entirely sure what inspired this. Maybe the thoughts of a lost friendship, the impending end of many friendships...
Sakura always felt grateful to Ino, how could she not? Ino had been her first friend, the first person to look past the big forehead and see a little girl who wanted a friend. Back then she'd been so childish, so easily hurt and saved by such trivial words.
Forehead-girl.
She'd been so happy to have someone to talk to, to turn to when the other girls picked on her. Ino always stood up for her, in fact once it became known that Sakura was Ino's new best friend, hardly anyone was mean to her at all.
Of course the new girls whispered about how it was just pity, how she was just her latest plaything in a long line of them. Sakura knew they were just jealous but at the same time she worried that one-day Ino would leave her behind as well.
Ino taught her how to be proud of her flaws. You get picked on because you hide it.
But on that day, she couldn't step aside. She loved Sasuke, and no matter how much she owed to Ino she couldn't give him up. She wouldn't lose to anyone —Ino— anymore. And for once she wanted to be chosen first instead of Ino.
Ino with her blonde hair and blue eyes, Ino with her clan name, Ino with her awesome ninjutsu and her plant knowledge. Next to Ino, Sakura did indeed feel like a thoroughwort.
Sometimes Sakura thinks she hates Ino, and she does but at the same time she doesn't. It's so easy to be envious of someone like Ino, but at the same time Sakura wants to be like her, wants to be confident and certain and beautiful.
So she gives back the ribbon—the sign of their friendship— because it's the surest way to break all ties with this girl who is her bestfriend-rival-hate. She sees the spurned look in Ino's eyes but can't help but follow through with this.
She doesn't want to be the one who's left behind after all.