Perhaps it's because Mikasa is so strong and threatening when they're out there, but shy and blushing when Sasha kisses her. Sasha knows she can never hope to outmatch her on the field, not in a million years, but at least she can feel special when they're all alone, just the two of them, and Mikasa her with the strong arms that have killed titans, and being together is all that matters in the end.
Perhaps it's because of the cute remnants of Sasha's accent that she can't help from time to time. Mikasa never judged her for that, why would she? Sasha knows it and sometimes, just sometimes, she lets her guard completely down around Mikasa and lets it all out. Mikasa loves and cherishes these moments, the way Sasha smiles just at the old memories undoubtedly coming back as her tongue loosens and the speech patterns she thought she had forgotten come back at once.
Perhaps it's because Mikasa saved Sasha's life so many times already, saved so many people's lives in so many battles. The ones who say she's emotionless couldn't be more mistaken. She's compassionate and caring and just because it rarely shows on her face doesn't mean she doesn't feel anything. And in fact, when she's in Sasha's arms and they're talking and kissing and loving each other, Mikasa's face says everything and more.
Perhaps it's because Sasha has never ever asked Mikasa to take off the scarf, not once, not ever. She respect her bond with Eren, the only family she has left. Yet sometimes, when she's around Sasha and feels very warm and comfortable and loved, Mikasa takes it off for a little while, and Sasha's soft arms around her neck make for a good replacement.
Perhaps it's because, despite her best efforts, Mikasa will never be as good at archery as Sasha. Sasha loves giving her some private lessons and teaching her everything she knows — not just because being around Mikasa is easy and fuzzy and comfortable and helping her through the motions is almost like hugging her, but also because she is a pretty good marksman and enjoys practising a skill the never thought she'd use again when she joined the military.
Perhaps it's because Sasha is expressing everything Mikasa isn't. She smiles, she laughs, she cries, she fusses, she says everything that's on her mind — and she'll always understand what's on Mikasa's. She notices when Mikasa is upset and she'll hug her close and whisper in her ear everything she needs to hear. She notices when she's happy — her eyes have that special twinkle — and she'll laugh in her stead. She notices when she's mourning and she'll hold her till whenever Mikasa is strong enough to stand up and go on with her life. Mikasa needs the support. It's hard to be strong.
Perhaps it's because Mikasa's hair is so black and tidy and beautiful compared to Sasha's own boring brown ponytail. Her eyes are grey and gorgeous and when she looks into them long enough, she feels she can see into Mikasa's heart. And when they spend the night lying on the grass and watching the sky together, her eyes reflecting the stars are so perfect that Sasha almost feels sad when Mikasa closes them to kiss her.
Perhaps it's because Sasha's lips are soft and pink and her smile is full of mischief when she takes Mikasa's hand to lead her God knows where — and Mikasa will follow her any place. Her eyes shine with every thought running through her and she is so cute and sweet when she starts jabbering that Mikasa sometimes can't help shutting her up with a kiss — it used to be with a piece of bread or a potato, and if she were a jealous person, she'd be upset that both options are equally pleasant to Sasha.
Perhaps it's because Mikasa jokes sometimes and no one ever notices but Sasha, and occasionally Armin and Eren. It probably doesn't help that she says everything with a straight face, but Sasha can hear that very subtle change of tone in her voice and see the slightest twinkle in her eyes, and when she laughs that twinkle turns into pride. Mikasa isn't very funny, but Sasha laughs every time just for that twinkle.
Perhaps it's because Sasha is stronger than she thinks. She is persistent and brave and Mikasa is proud of her. When she tells her, Sasha will wave it off and scoff at her. Mikasa still tell her every time she feels it. Sasha sometimes gives into despair, but she gets back on her feet with or without Mikasa's help. She keeps fighting.
Perhaps it's because of something else entirely. In the end, Mikasa and Sasha have a thousand reasons to love each other.