This is a great speech given at Stanford's class day that talks about how humans are similar and unique when compared to all other animals. The whole talk is really interesting and funny with cool examples of our evolutionary heritage, but it is a bit long (37 min). If you don't feel like watching the whole thing then at least skip to around min 27 where he talks about our dopamine reward system and how it may have implications on why religion developed from an evolutionary point of view. It may shed some light on why humans can be so irrational. The talk starts at 5:27 if it doesn't jump to it automatically. Hopefully this works, this is my first time embedding a video.