The vast majority of humans are decent people. Not perfect, but try to do good as they understand it. The genuinely amoral and selfish are a pretty tiny number all things considered.
Sorry, but we all have the selfish gene, right?
humans do good things for selfish reasons, pretty much constantly. I want people to be nice to me, so I'm generally nice to people. In my experience it works. Sometimes there is no way to avoid people who are not acting nice and then it's ok to be less than nice. Even then it's generally better to remove ones self from that situation than to be not nice in return.