I see two major things that answer this question:
The first is that Simba and Nala are willing to go to life-threatening adventures together, such as to the Elephant Graveyard. And there, once they met the hyenas and ran away, Simba chose to risk himself by running down to help Nala and swipe hard at Shenzi, even though adult hyenas would have a field day going up against a lion cub. With the odds against him, Simba ran down to save Nala.
And the second one is after the stampede. Simba's "death" hit Nala very hard, more than if the two of them were not friends. And after they reunited, everything Nala had to go with were her cub-hood memories of her and Simba. And even with just those, she still felt that strong bonding towards him even seconds after trying to kill him.
As for an actual crush? Perhaps. The only time we get to learn if they do *like* each other or not is when they are reunited and the "Can You Feel The Love Tonight" scene, so there is not much of an argument there.