Mikasa knows there are different types of love.
There is family love, the kind of love that is unconditional, a selfless love beyond understanding and that was the way she loved her parents, and Carla and Grisha, the way she loves Armin.
There is the friends love, the kind of love that is funny and caring and that is the way she loves Sasha and Jean and Connie and many more.
And then there is the love that is unconditional and selfless and funny and caring all at the same time. The kind of love that makes her reckless and desperate when he is in danger and that is the way she loves Eren.
She has many opportunities to tell him that she loves him, but she never does.
She says other things, like 'Don't die' or 'Thank you'.
She thinks that he understands.
So when he tells her that he will be there for her forever, as many times as she needs him, she knows.
And when he whispers on her ear how he doesn't want to lose her, how scared he was and 'please don't do it ever again', she smiles and says 'I love you'.
They don't need more words, they both know.