The Great Filter

So what acts as the Great Filter? Why haven't we been contacted by alien civilizations?

My Guess is that at least one of the following applies:

1. Life is rare.
2. Intelligent life is rare.
3. Intelligent life is rare in environments having physical resources for interstellar actions. (Intelligent ocean life has a difficult time developing interstellar technologies.)
4. Most intelligent life stinks at moral reasoning and action, leading to takeovers by psychological and evolutionary egoists, preventing civilizations from ethically and technologically developing the ability to colonize or communicate long distances. This has already happened numerous times with human civilizations. Internal egoism and tribalistic outgroups destroy the civilizations. 
5. Technologies of nihilism or hedonism prevent sufficient advancement and interstellar activities. (For example, a few individuals in advanced societies can create bioweapons and other weapons of mass destruction.)

On the plus side, at least we haven't been destroyed or conquered by robots from elsewhere, yet.