I often hear people say, "I like racing games" or "I'm more of a shoot 'em up kinda guy". While I don't dispute that at all I want you to try to remember your five most fulfilling gaming experiences. If you're a long time gamer you'll find yourself with five very diverse games with little in common. What they will have in common is that they were all one of the first games you played in that specific genre. The human brain is tuned to reward you when you master new skills. Repeating the same things over and over does not grant the same sensation.
Back in the days me and my friends were completely absorbed by Tony Hawk. Once we got the hang of it we couldn't get enough. We played for days on end. By the time Tony Hawk 2 hit the stores we were so excited we nearly wet our pants. We gathered up to play it, crawling out of our skins with anticipation, but the feeling wasn't there. It simply wasn't as fun. In all fairness, the game isn't at all bad. Probably better than the first one. But it was more of the same thing and the interest faded after that.
Getting into new genres can be scary because you're on uncharted territory, but once you get past the obstacles the feeling of mastering something new vastly outshines utilizing your old skills. It should be pointed out that the exhilharation may not come the first time you play the game. It may not even come the second or third time either, but when it comes it can never be repeated. This new experience reward is a one time fix. You will either have to put up with the diminishing effects or find yourself a new game.
I warmly recommend Ralph Koster's book A Theory of Fun that digs deeper in to the subject.