I agree with Twisterdog that this is probably to small a sample to be a true representative.. but it is interesting..

Working at Vet clinics for almost 9yrs... I can agree, I've seen many many aggressive Chis. Like many other people have stated, mainly because the owners think their behavior is cute and just let the dogs get away with it. I've also seen a lot of Dachshunds that were a bit aggressive, but also pleanty of sweet ones... Min Pins as well.. and many cockers..

But I also agree with what Giselle said about aggression and reactivity. My smooth Collie, Paris, is a very sweet dog.. a giant lap dog... she does great with other dogs in the house or out in the yard, or at a dog park or other large fenced area.. but she does have A LOT of chase instinct/drive and has issues with wanting to chase cars if given the chance (her grandmother and great grandmother - neither of which I owned, both were owned by different people I knew - but both had the same problem..both were hit and killed by cars they tried to chase)... also if she is in a small fenced area (think kennel run etc.. 10 x 10 or 10x20 or some long narrow run) for even just a few minutes she gets very "fence aggressive" and will fence fight with a dog next to her.. and run up and down the fence barking and acting stupid... which is why if I ever have to go away for a few days I try to take Paris with me so she isn't in a run going crazy and acting stupid... if she was kept in a kennel environment for long I think she would become very cage aggressive.

Also if Paris is in a kennel/run with another dog and a car drives by because she has such a strong chase instinct she gets so crazed and excited wanting to chase the car she will turn and jump on and nail the dog in the run with her.... however if she is out in an open area... she is perfectly fine...