Why do employers try to find fault?

I work 2 jobs and am very contentious at both. I'm getting so tired of the fact that when we have a monthly performance meeting they are picking the most minor things to critique.
Why do managers do this? Today after telling me that I had 86 good reviews this month she ends with telling me that I need to sound more confident because I'm conveying hesitancy. Well Im being recorded while working and Im trying to do my best. Sheesh