It is something TOTALLY NECESSARY, the difficulty is what we can easily like or dislike about a game, I mean, an easy game will be an easy game, maybe we like it, but we will never love it, a game with a "perfect" difficulty, we tend to like it 100 times more, apart from the fact that it becomes a personal challenge, we enjoy it much more.
RE: Difficulty and its vital reason for existence [ESP/ENG]