Why?
She could ask herself why: why was her planet destroyed?—why did her family perish?
He could ask himself why: why was his planet destroyed?—why did his wife perish?
There are no answers to these questions, nor the question of why they survived, why they're the ones training adolescents to protect the world, why they carry the burden of being the last of their kinds. There are some questions which they can answer, though.
Why does she love him? Well, there are many reasons.
She loves him for his kind heart, his tenacity and benevolence. She loves how sweet and loving he is, yet always properly in command. She loves his ability to take control as a natural leader, yet he never oversteps his rank. She loves how protective he is, of his own, of his friends, of his team. She loves his smooth, blue scales. She loves how deep his eyes become when he looks at her. She loves how pointy his ears are. She loves how strong he is, how safe he makes her feel. She loves everything about him.
Why does he love her? Well, there are many reasons.
He loves how kind she is, tenacious and benevolent. He loves how comforting her naturally commanding presence is. He loves the grace she possesses unwittingly, and the majesty with which she conducts herself. He loves her genius, and the way she uses it for the greater good. He loves how adorably stubborn she can be, especially when protecting her loved ones. He loves those dangerously green eyes of hers that shine at him. He loves her pearly fangs, and her perfectly pink lips. He loves the speckles running down her cheeks. He loves her ears and the way they stick out of her mop of dark curls. He loves how strong she is, and she doesn't even know that she's why he can still find strength in himself. He loves everything about her.
So, the final question: why can't they freakin' just ask each other out?