Listen closely the next time someone voices his opinion on something. It will often be based on a single, personal experience. For example, "I once owned one of those and it never worked." He has decided the product is flawed, now and forever. Or, "She is unreliable because she forgot to do something for me a couple of years ago." This implies she is chronically unreliable, and will continue to be so.
How quickly we judge. How little thought goes into most of our choices.
If you find yourself forming an opinion based on one instance, try this: Think of a similar situation--without carrying over any bias--and compare the two. Tell me if your first opinion holds up.