I ship Eusaria because of their similarities in character and life experiences, their desire to protect Mondstadt despite their traumatic and dark pasts even at the cost of their personal happiness. Both are outcasts and thus the only ones who can truly understand each other and they are the opposite sides of the same coin (huge Cipher and Pixy vibes from Ace Combat Zero which is my favorite game and even their designs contrast each other). In short, Eusaria is a very dark, complex and versatile ship.