Eren watched two of the last people he needed in his life die. With them, he lost his sanity.
If you have not seen the most of the anime, Attack on Titan (Shingeki no Kyojin) then you need to go right now. Get out, and do not return until seeing at least twelve of the episodes in order. There will be references to the show, and what happens at some points in the anime.
Psycho!ErenxLevi. I don't even know what's going to happen here, so, that's that right then and there. Boyxboy... No hate. Except for the prologue, each chapter will be short, one page at least, though. Always at least one page long.