That's one problem I have with theology and belief systems that say that people are born naturally 100% good, nice, sweet and innocent. In my experience, we are born bundles of conflicting, often selfish desires and we have to learn (painfully) how to channel those tendencies productively and nondestructively.
I've learned that personally it works best for me to have low expectations so I can be pleasantly surprised. I tend to come across as a kind of an up-beat pessimist. Definitely not in the rose-colored glasses camp.
However, I've heard optimists get more done even though they typically have a less accurate conception of reality. Go figure.