Several of the people I knew from high went to the same college as me. One day I was talking to a friend about another friend of ours from high school.
I said, "I know Jill and I were never really close in high school, but here at college she doesn't want to have anything to do with me. Everytime I see her, she totally blows me off." At this, my friend started laughing uncontrollably. "What?" I asked.
"Todd, she says exactly the same thing about you."
So the next several times I saw Jill on campus, I made sure to be extra friendly. Sure enough she returned the friendliness in kind. Whenever I catch myself making negative assumptions about someone else, I remind myself how wrong I was about Jill.