There's a reason stereotypes are stereotypes. Most of the time, it's pretty accurate. Obviously, not every pitbull is overly aggressive, but it's well known that people use them a lot for fighting. Quoted from wiki:

The term pit bull refers to certain breeds of dog – namely, the American Pit Bull Terrier, the American Staffordshire Terrier, the Staffordshire Terrier, and any crosses between the three. In a few parts of the world, the American Bulldog is also classified as 'Pit Bull'-type dog, despite the fact that they have major genetic differences.
The American Pit Bull Terrier was bred for working terriers and eagerness despite the threat of substantive injury, strength, and athleticism. American Pit Bull Terriers constitute the majority of dogs used for illegal dog fighting in the United States. Widely reported pit bull attacks have resulted in the enacting of breed-specific legislation in several jurisdictions, as well as increased premiums for liability insurance.

So, please don't pretend that all pitbulls are as kind and sweet as yours.