But if we have not learned, we are destined to repeat ourselves over and over. A do-over will not make one a new person, but simply an ignorant one destined to repeat themselves until the cycle breaks. When Joel wakes up from the procedure, he simply goes back on the train and meets Clem again. If you restart the movie after finishing it, he wakes up the same way he does at the end, runs to the train and begins his relationship with Clem all over again. And you’ll notice the pair have the same conversations a few times over, denoting multiple timelines where they could have been together in the past twice. So they could have had their memories erased many times over. Years spent together and then erasing each other. Whoa. Imagine that?
Article source: https://www.cinemablend.com/news/2552601/eternal-sunshine-of-the-spotless-mind-ending-explained-what-happens-and-what-the-lacuna-tapes-mean