Reason one: They are compatible. They're personalities are quite similar and they have both gone through hell to get to where they are. The whole thing about shipping in the Yaoi stage is that a girl will ship people with the aspects she would like in a man. When those things are combined, they will be attached to it.

Is it wrong to ship Eren and Levi?

Eren is currently 19 in the Manga and Anime, with Eren being of legal age most believe that the ship is legal however, some still believe it's wrong to ship it as Eren met Levi when he was 15. ... Eren could just as easily overpower Levi as he could him.

What is the most shipped couple in AOT?

When it comes to Attack on Titan ships, Jean and Marco often rank among the most popular pairings. A quick Google search can confirm that a large number of anime fans are on board for these two (or they would be if Marco hadn't perished way back in season one.)