The challenge of Bloodborne has proved to be incredibly popular with gaming audiences. While some predicted the hardcore nature of the game might be too much to overcome for a mass audience, that game has gone on to surprise even Sony with how well it did.
But what has kept people coming back? The director of the game, Hidetaka Miyazaki recently talked to Edge (via GamesRadar) magazine about he to is makes the game work:
I think being able to comprehend something contains an innate fun factor. See, being able to comprehend something after putting in the required effort is conducive to a fun 'Yes, I get it now!' kind of effect. If you didn't have to work for it, the experience would be far less stimulating.
This is the same logical framework powering such as difficulty level and sense of fulfillment. To feel fulfilled, you must first have a goal that needs fulfilling. At the same time, it must actually be possible to fulfill said goal.
The game is absolutely predicated on that sense of accomplishment that comes with slaying the horrors of Yarnham. There is a very interesting gameplay loop that kept me going in this game and that is a testament to its design.