Dark Souls is a reaction to modern game design and modernity in general. Its difficult in an era where games are designed to be finished by anyone. It requires total concentration in a world where we usually have our eyes set on multiple screens at any one time. Dark Souls hardly makes sense as a modern video-game and thats what makes it so great.
Much of the difficulty in Dark Souls is born out of the repetition that the game requires. An enemy will kill you numerous times but in doing so you learn the attack patterns and the strategies to eventually defeat it. From each death there is a sense of improvement, as you become better acquainted with the enemies move-set and starting doing more damage to their seemingly endless health bar. This stops it from feeling banal and tedious. However, each attempt at a boss fight requires the player to traverse an area over which they have already triumphed. The urge to get back there quickly will often lead to running past enemies on the path and rushing, which can lead to more deaths. This leads to the player wanting to rush more and so being more likely to die.
Dark Souls is not hard just from being repetitive, its also difficult as it is unflinchingly punishing. A single mistimed roll or a missed attack will almost always result in a large chunk of health being taken from you. Yet, this never feels unfair. Partially, because it is typically due to your own mistakes, but also because, when at full health, the vast majority of attacks are survivable. There are a few exceptions to this, but mostly making a mistake is something that can be recovered from with a swig from an estus flask. Doing so is a tough task as there is the added tension of knowing that a single error will lead to the dreaded You Died screen. The timing of when to use the estus flask is then crucial as there is a desire to use it immediately in order to have the comfort of a fuller health bar, but this often leaves the player vulnerable to another attack. Again, this is Dark Souls trying to stop the player from rushing and to slow down.
Furthermore, Dark Souls requires constant attention. The game cannot be paused in the middle of action. There are areas that are designated (the bonfires) as being safe, these are infrequent though. Every corner poses a threat and so the player must constantly be on guard and prepared. It is not only enemies that are a danger as there are numerous environmental dangers, ranging from precarious ledges to the cacophony of pressure plates and swinging axes that inhabit Sens Fortress. Giving Dark Souls anything less than your full attention will invariably result in death. Glancing at a phone screen may mean an enemy can get one devastating attack living the player on the precipice of death. The pressure ratchets up, the tension rises and a simply fight has immediately transformed into a near death experience. This absolute, undivided attention that Dark Souls constantly demands is hugely opposed to half-attention that most media requires.
You have to give your all to Dark Souls to even stand a chance.