It's a matter of preference, honestly. Hating people is just like hating anything else.
I'm a bit misanthropic, but I don't automatically hate anyone. My opinions are based on my observations; anyone can prove that they're a good person. However, I'm a lot easier to sway in the negative direction.
As to why I'm so negative over being positive, from my experience everything is far more likely to go horribly wrong than succeed, especially with people. People are more likely to destroy than to create and very few appreciate the beauty in the world. People tend to be apathetic, self-serving, destructive, greedy to the extreme, hateful and overall not good. There are those who aren't, but I see people who are frequently.
I don't hate people for being people; I hate them for doing the things they do. I am noticeably biased toward the negative because of the things that I see every day, but people are not necessarily evil.
I guess it's a little bit of envy, too, but I'm not gonna go into that right now.