Let’s talk about this.
Mikasa doesn’t really smile, does she?
So why now?
I’ll tell you why.
She felt, truly, deeply in her heart, that they were going to die. With their last moments alive, she didn’t want Eren to worry over being helpless and weak in those moments.
She didn’t want him to blame himself. She wanted him to realize that he had helped her, that he had changed her life, and he wasn’t useless.
That if they were going to die, they were going to die together. In their last moments, she wants to show her gratefulness, her love, her smile. That was what she wanted him to see if they were going to die.
Not her tough exterior but her inside. What she holds back from showing, she wanted him to see in their last moments alive.