time doesnt work the same in aot in this world. the future could effect the past and again the pain could affect the future sso its kind of a loop. that a short bague explanation i recommened seraching this on youtube because they have a very good and detailed explanation for this than i can rovide myself.
https://youtu.be/RKBXj96MYmM (anime only, still a detailed start to understand it)
https://youtu.be/BTfH5C15fPA (manga chapter 121, confirmation)
the manga is obviously still not finshesd so this is not a full exlanation but you can branch of from these videos and find new once if you dont ffully get the theory by then.
The manga's gonna end on Eren woking up wondering why Mikasa's hair are so long sjwjsjwksisjs
someone please explain to me about the timeloop theory