That thought, after it's contemplated, refuses to leave her head.
Friend. A position in Anna's life that's been vacant of anyone but Kristoff.
Anna is certainly able to admit to herself that she likes Elsa's company. She likes Elsa. She's glad they met, but not of the events that surrounded the occasion. It's for that reason she can't tell Elsa.
No matter how much she wants it, there's no guarantee Elsa wants to cross the bridge into friendship. For her, it's just an arrangement.
That thought hurts more than it should.