Well, for one thing, it's a lot more than driving in a circle. They do drive in an oval by the way, not the circle, and they also actually do make right turns- and the top three NASCAR series' race on road courses a few times each year, road courses being tracks shaped like this one in Sonoma, California: http://www.na-motorsports.com/Tracks/CA/images/sears-point.gif It has left and right turns.
And there are a ton of other different shapes of tracks that NASCAR and other motorsports race on.
And you just said one thing that makes it entertaining "...really fast cars...". How are cars going 200 MPH not exciting? If you call cars going that fast boring, it's illogical. I agree, it can look boring on TV sometimes, and TV doesn't do it justice. If you go to a race in person, you get to experience the speed, sound, and excitement. It may just look like a bunch of cars driving in a circle on the outside, but there is so much technical stuff involved on the inside that makes people within NASCAR know that there is a whole lot more to it than driving in circles. It's much more complicated than that. It's fun to watch people race hard against each other and try to run into each other's cars so they can take the lead and win the race, it's fun to see someone make daring moves to simply gain a position or take the lead, all while focusing on driving the car at 150-210 MPH! It takes a lot of skill to be a successful driver and isn't nearly as easy as driving your typical street car. Even if your street car drives in more than just circles. NASCAR and ARCA drivers drive on some of the hardest circles you'll ever drive on, and the fastest ones too. It's a whole lot of fun to watch super fast cars fly by you in person while racing each other with intensity.
If you're still not entertained by ovals, I'd suggest watching other motorsports that race on those road courses with left and right turning ,that I mentioned, more frequently than NASCAR does. Those other motorsports organizations include: IndyCar [the super exciting Indianapolis 500 is their next race on May 26th, where 300,000 people come to watch each year] (http://www.indycar.com/), Formula 1 (http://www.formula1.com/), Grand-Am (http://www.grand-am.com/), American Le Mans (http://www.alms.com/), Australian V8 SuperCars (http://www.v8supercars.com.au/), NASCAR Euro Racecar (http://hometracks.nascar.com/series/euro_racecar), and the NASCAR Canadian Tire Series which races on a lot of road courses unlike most NASCAR series' (http://hometracks.nascar.com/series/canadian_tire_series)
And yes, all of these types of auto racing are indeed sports, known as motorsports. They all take an unbelievable amount of physical fitness and are all very competitive. For proof, check out this video that explains the physical activity that takes place while NASCAR drivers race: http://www.youtube.com/watch?v=ZkKKMI9laIU
I'm glad I could educate you about NASCAR and all the other great and exciting motorsports!