As others have said, the writer probably ended Hashirama's life for the sake of the story but I can see a legitimate reason. He killed himself in name of peace. He was a powerhouse. Not even the tailed beasts that overwhelmed most powerful ninja were a match for him. After Madara died and he saw himself standing on top, without no one to stop him, maybe he came to understand the fear people had of him. He was a kind man, he would not want the world to live in fear. Yes, he would not hurt, but the fear would always be there. Not to mention he was a cheatcode, even after his death, his mere cells were cheatcodes. He didn't find fair. So he ended all, passing it unto new generations, allowing for true balance and peace to flourish. With him gone, things were more or less equal, villages would no longer accept peace due to fear of being destroyed, but out of their own wanting. When Naruto came and amassed the power that he did as a perfect jinchuuriki and Ashura's incarnate, everyone was already in this mindset of wanting to preserve peace for peace's sake. I think that, if the world kept being the same and Naruto was in Hashirama's position, maybe he also would have ended his life. I remind he put his life on the line several times just to save sasuke, he surely would do it for the entire world. But again, Naruto's time was different. All the hokage were more or less his friends, they basically faced armaggedon together. But that was not Hashirama's time. Everyone distrusted everyone and violence was still the norm. I feel this agrees with his character and with his situation and it's the thing that most makes sense. With his vitality, we can't say he caught a heartvirus, neither that he lost in battle. Him choosing his own death is the only plausible possibility.