Board Thread:Theories/@comment-30785465-20170219162046/@comment-27441457-20170219235800

That's definitely the cause of their cravings for humans, even Hange theorized this in Chapter 57. This also explains why Titans will focus on a shifter when one is around rather than other humans nearby. It's a very rare possibility that a human will be a shifter, but seeing a shifter right in front of them gets their attention immediately (ie Eren in Trost).