We all assumed that Mikasa's dream was her hallucinating but it was actually a way for Eren to confess and show her that even if they ran away, it would have never worked out between them because of the world they live in and Eren's responsibilities. Eren’s death is pretty sad when you take this in consideration. We can see his selfish desire to remain the only man in Mikasa's life, even after his death. So when Eren sees Mikasa's enter the Founder's mouth wearing his scarf, he smiles and know that she will not forget about him. And Mikasa finally knows that her love was reciprocated.
The final panel of a thin Mikasa spending her free time by Eren's grave is freaking depressing.