Sparity is such a strange ship to to me. It's almost certain that it wouldn't happen. And if events transpired differently no one would probably care about this ship.
Maybe it's the fact that it's almost impossible, that makes the ship so intriguing.
Sparity if it ever happened would only be possible once Spike really grows up and becomes his own person.
But I like to think Rarity isn't bothered by Spike's crush and she would rather let him decide whether to keep crushing on her or not instead of telling him. Maybe because she thinks its cute and even respects big nearly impossible dreams like that. Perhaps it reminds her of her own fillyhood.
Just my theories.