Challenge and Reward In Games

I was playing computer games again, and I started to think about 2 things.

Challenge in Games:
There's a certain crunch point in most online games such as Counter-Strike Source, or America's Army, when you know you're so good, that no one can surpass you. At that point, nothing in the game is a challenge to you. Everything is second-nature. You know every strategy and every tactic, and they work every time. Every time you join a server, no matter what map, you totally kill everyone every time. Challenge = Gone.
It's at that point the game starts to become boring. Why? Because there's no challenge. Sure, it's fun to "own" people, but after doing it so many times, it becomes all the same.

So my conclusion came to be, a game must be continually challenging for it to stay interesting. This applies to single player games just as well. If you can conquer the gameplay mechanic in the first five minutes of the game, then the remaining 15 hours of the game will be boring. A good game will continually challenge the player, forcing them to become better at it until the end.

Reward:
It has been virtually stated that games need to reward the players, otherwise the player will probably lose the incentive to keep playing. If Half-Life 2 just started you out with a pistol and crowbar, had you kill 100 zombies, over and over in different locations, and then it ended, there's no reward there. Instead, in the real game they give you more weapons (reward), a little bit more of the story (reward), cutscenes (reward), and new locations (reward).
There's only so much a player can endure before he's tired of playing the game, seemingly going nowhere. That is why the player needs to be rewarded in some form or manner.

Conclusion: Play different kinds of games.