I am wrestling w/this line of thought these dayz as well. I think it means (in the case of the example you put forth) that within your worldview, that person was capable/ perhaps even probably or most likely (even if just in the case of one's "bad luck" and poor choices) capable of being acting in the manner you deemd "mean".
When you believe the best about people, no strings attached, you will likely experience the best of people. When you believe that others are not capable of, or probably wont act in a way that is good and beneficial to your world, they will likely accomodate your "belief" with their actions/words. Its probably more complex than my simple response, but I think it is the gist of the teaching.