How does Orion and Darcy fell in love?

So I'm starting Book Three and got to the part when he sends Darcy a message saying he doesn't want to hold back with her... But I do really have this doubt like, at this point in the story they have a crush (I mean all the ladies have a crush on Orion) buuuut why he has this interest on Darcy? Why not Tory? Like, they're twins and similar haha...