If he falls in love with a Companion, a creature of the Alliance, and he loses a part a part of his independence. Becomes a sellout, truly surrenders to them, and that's when the war is finally lost.
If she falls in love at all and she can no longer be a Companion. What is she without her job? That's is what defines her, where she exists in the world, how she always knows her way even when she is lost. Without that, she doesn't know where she'd be or who.
They fight each other because the fight within them cannot stay contained for terribly long and surrender is not an option.
He'll fight for her defend her honor against those who do not respect her, though he himself doesn't respect what she sees as one of her defining characteristics. She'll cry for him, lecture him, roll her eyes, and hold him as he bleeds. The moments of weakness tear them both apart.
They are the parents of the family that is the crew, but they are estranged from one another. They cannot get past the surface conflict to what is underneath. The hard shells block them out, the barbs spark fights, the burrs of insults get stuck in hair and they can't help but lash back.
If it were anyone else, both of them would've stopped fighting long ago. Not even picked it to begin with, but there is a spark they can't ignore, even if they are going to attack each other with words. They can't help but fan the flames.
She is drawn to him for the mystery he exudes. He is frustrated by the fact that she is complex creature.
The long looks, glances that speak a thousand words, a brush of the hand against the arm, those are the closest things to intimacy they may ever have.
She can't rebuild him. She doesn't want to. He can't change her. He doesn't want to.
It's all just an elaborate unspoken dance and they are the partners, the lovers in arms, and that is how it must be.
Until the music stops, and either they change partners or confess. Letting go is painful, heartbreaking, but confessions just might break them both entirely. The aftermath would be strange, but they would have each other.
But for now, they dance on.