It was a "game" that we sometimes played.
When we meet each other again after being reincarnated, we pretend that we are strangers. We pretend that we don't know each other, even though we have thousands of years' worth of memories together.
Neither of us can really remember how, or when, we first met. Probably like how a normal person recalls their childhood, our memories from our early "lives" are a big mush, and we often have trouble telling them apart.
But without a doubt, we will always meet each other in each of our "lives". Because every time we were reincarnated it was like living in a different world, our relationship also changed with each reincarnation. In some "lives" we grew up together, in others I didn't meet him until we both already had grandchildren. In some "lives" we were family, and in most we were lovers.
Well, not that it is a topic that comes up easily, but neither of us have met anyone else who has memories from all their previous "lives", like Eren and I do. That's why, after so many millennia, it has become a "game" for us.
And because of that game, it wasn't until I saw Eren crying that night, that I realised… I was the only one in this "life", who remembered our past "lives".
