Seth Godin's post, When data and decisions collide, inspired Generous Husband's post on the difference between what we think is true, and what is true.
In short, what we perceive to be the truth becomes reality for us and we respond according to that reality. If I believe someone is being dishonest with me, I will treat them with extreme caution or scepticism, even if they're actually being totally honest.. If I believe someone doesn't want to talk to me, it will affect how I relate to them, even if my perception is entirely false.
We can't go round challenging ourselves on every last thing we believe about people or situations, but perhaps it would help to check our sense of reality when we find ourselves believing things about others that may not be true and for which we have little real evidence. In my humble experience the truth we tell ourselves can often turn out to be false.