Board Thread:Manga/@comment-26495049-20161201065442

I have a theory about why Grisha did this. Maybe becoming a titan shifter affects the human physique so much that after some time, I'm guessing about 20 years, that the human body would no longer be able to cope with its damaging influence, causing them to die and their power along with them.

This would motivate titan shifters to find a successor to ensure this power survives. It would explain why Grisha, who was probably approaching the end of his titan shifter "lifespan", would want to sacrifice his life to pass his power on to Eren, since he is about to die anyway.

Similarly, it would explain why Kruger would want to pass his power on to Grisha, even though Kruger himself is by far more experienced in titan shifting. He probably needed someone to infiltrate the walls for an extended period of time, and being close to the end of his lifespan was unable to do it himself, causing him to actively seek out a successor(Grisha) to do so.

Reiner and Bertolt's strong desire to complete their mission and return to their hometown is probably motivated by this too. Knowing that they don't have long to live, they would want to spend as much of the remaining time they had left with their loved ones as possible, before they are sacrificed to another warrior to ensure their titan powers survive.

The only exception to this rule is if the shifter is of royal lineage (like Zeke and Uri Reiss), in which case their genetic makeup would make them uniquely suited for titan shifting, enabling them to live out a normal human lifespan.  