Why is it that people assume the worst of another person?

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Just recently a friend of mine decided to believe that I said something that I didn't. What really bothers me is that my friend never even came to ask me if it was true she just got mad and has been avoiding me. When if she truly just came to me and ask me I would have told her that I didn't say it and then I would have confronted the person who told her this. I'm tired of it always being me going to my friend to get things straightened out when she doesn't even bother to come and ask something so simple as did you say this. Would a true friend do something like this? Should I just avoid her too? Should I just walk away?