I enjoy challenge. You probably do too. Without challenge their is boredom and routine. However, I have to qualify this. For challenge to be enjoyable:

1. it has to be solvable, or atleast appear solvable
2. you have to make progress every so often. be thrown a bone or two.
3. it has to be something that puzzles not a challenge because you are doing things the hard way or with an inadequate toolkit ie) rebuilding a car engine with only a toothbrush is not enjoyable