Faith isn't good with words. She’s known for it. It’s fine; usually, actions work just as well.
Not with Buffy.
Faith takes Xander and says: I can be better than you. She takes Riley: I can be you. She takes Angel: I can be more than you.
Buffy doesn't hear any of this. She hears only, ever, I can be the worst of you. A note held at the tip of her nose so it blurs to gray. She and Buffy are too close to read each other.
Maybe Buffy said I love you back, and Faith didn't hear it.