Impossible, because I love a good challenge. However, I usually play games on the easiest setting first. Then the next time I play through them, I'll move up to the next setting, then the next, and each time I replay the game I'll move up until I've completed it on all possible difficulty levels. Then after that I'll just stick to whichever setting I enjoyed the most...
For example, before it was stolen early this year I was halfway through a play through of Gears of War on the third difficulty setting. That said, I haven't played it since getting a replacement from the insurance, purely because I've been too busy trying to finish FFX-2 100% (I only have an Xbox 360 at university, the PS2 belongs to the family and stays home, so I'm trying desperately to finish it this week in between training for my new part-time job and doing jobs around the house while we redecorate...).