Difficulty settings in video games serve a very important purpose: essentially, they grant less experienced - or less skilled - players access to the later stages of certain games, without forcing them to spend weeks or sometimes months honing their dismal abilities. Games like Devil May Cry and Ninja Gaiden pride themselves on their often excruciating difficulty levels, so it's only fair that less able players are given the option of making things a little easier for themselves. After all, they paid for the games, they have a right to experience as much of them as possible.
Unfortunately, some developers don't feel the same way, taking every opportunity possible to berate, taunt or otherwise humiliate their players for taking the easy route. In some cases, not only will a game actively antagonise the player, but it will restrict content, forcing them to replay huge chunks of the story or even deny them the satisfaction of a worthwhile ending.
To put it bluntly, some games hate you, and if you don't show them the respect they deserve, they'll let you know all about it...
The Ninja Gaiden games are notorious for their difficulty; in fact, its one of the central appeals of the series; so much so that an easy mode option has rarely been included in past entries.
In Ninja Gaiden Black however, an easy mode was begrudgingly included by the developers in order to satiate their western audiences, who were seen as a less competent audience than their Japanese counterparts. In Ninja Gaiden Black, a new mode was included entitled Ninja Dog, a mode which made the game considerably easier. It also forced you to wear a very pretty purple ribbon for the entirety of game.
Oh, and Ayane (the game's secondary character) will verbally mock you throughout the entire game for your weakness, scolding you even in victory.