Video games seem easier than they used to be. Sit an old gamer down and ask them about the games in the arcades and the early home consoles and you'll be subjected to war stories as heartfelt as those from any veteran of real wars.
Certainly there are games that are very easy and those that are incredibly difficult. The difficult games are frustrating and the easy ones are boring. I don't mean that games with simple mechanics and rules are boring. Tetris is a perfect example of a game with simple rules that gets very challenging. No. I'm talking about games that don't provide enough of a challenge.
It's not just a matter of making a game challenging. The player like to feel good enough to move forward and keep going. Barely being able to cope is challenging but not very satisfying. A good way of looking at challenge is an article from Pixel Poppers called Real Games Have Curves: Welcome to the Competence Zone.
Next time you find yourself unable to leave the computer because "there's just one more level" or "this time I'll get it perfect" blame game designers who managed to get the balance and challenge right.