Here are some explainations by tmujir955 on YouTube
https://www.youtube.com/watch?v=YdmUjrR7D0w“Some people really did not understand the ending:
The ending is really good.
1. Ymir was in fact in love with king fritz even if she was abused. Remember she saw someone kissing and thought this was what love felt like. So when King Fritz decided to make her his woman, for the first time she thought she found love. That's why she was so loyal with King Fritz even for 2000 years. And that is also the reason why she found peace when mikasa killed eren because of love. She saw in Mikasa that love is doing what is right and not just become a slave of one's lover even if the lover is already doing something terrrible.
2. For eren, it is not totally out of context, eren was never the level-headed guy since the beginning, he was always rash and driven by emotions. Apparently, when he saw that people of paradis will die by Marley's attack (after he kissed Historia's hand) he was driven by that desire to save his friend especially mikasa and armin. It all boils down to a circle why eren kruger said that to save armin and mikasa you have to break the cycle. So he was suffering alone, tell me if someone sees that everyone you know will die of an impending attack what will you do?
3. Farmer kun being the father was already mentioned before, it was only delusional and toxic fans who think erehisu is a thing made a big deal out of it. Historia already fulfilled her role in Eren's plot. The reason why eren told grisha not to kill rod reiss is because historia will not come to the picture and make him see what will happen in the future since only historia is the remaining member of the royal family who can activate the power of the founding titan which reside inside eren.
4. Eren allowing Dina to attack his mother was also for the cause of saving humanity. You may think that it goes against his character but understand that eren's desire to save all of paradis outweighed his desire to save his mother I know its awful but he was also the dark horse of aot. Something that is already expressed in eren's unstable character.
5. Eren was, for once, selfish when he told Armin that he does not want Mikasa to love other guys. He did everything for everyone and not for himself. Honestly, this ending is one of the best. It justifies love in this cruel world and one's own selfish desire.
People expect eren to ONLY be a hero but he is also human. Like any other human he loves someone and wants to be with her. It made me cry.”
“Yes! When Ymir sees that Mikasa is able to let go of Eren and kill him, she is also able to let go and that's why she is smiling, and why Mikasa is the one that Ymir chose. As for Eren he is legit a slave to Ymir's desire for freedom and everything that happened, even him killing his mom stemmed from that. It's honestly extremely sad that Eren struggled the hardest for freedom but was the biggest slave of them all.”
Honestly I’m not that happy or disappointed about the ending. You can say that I’m relieved that the my beloved characters are alive with their family. To heck with the world’s population. I love this manga. The ending won’t make me forget the rollercoaster I felt while reading this, so thank you, Isayama. But still. Sad twerking mikasa and eren my heart uwu. I had the same understanding with the comments. but I can’t really explain shit since I’m very magulo. (disorganized)
;((( okay time to go to therapy.