im beginning to believe he might have killed himself to preserve the peace between the Senju and Uchiha. after his final battle with Madara. he believes Madara has died. and to prove his feelings he does the impossible and sacrifices himself. asking Tobirama not to bring him back with Edo Tensei and to treat the Uchiha well.
it would follow the trend of every hokage sacrificing themselves to protect their village and its people as every other hokage has done so
maybe this page is hinting to that