Good point PrimateDave about basing beliefs on sci fi.
This however is the point of view that most astronomers and astrophysicists have come to.
1) Microbiological life is common throughout the Galaxy.
2) More advanced life like vertebrates is rare.
3) Sentient or intelligent life is very rare.
4) The earth is not an example of what's going on in the Galaxy but an exception.
The reason it is an exception is because most stars (and their planets) are at the core of the galaxy where they are scrunched up real densely at an average distance 10 times less than our average here on Earth which is located in the spiral arms of the galaxy. This proximity to each other creates a tumultous environment where microscopic life has a problem evolving into sentient beings.
The reason for the tumult has to do with supernova explosions and cometary bombardment being more destructive over there in the core, where most stars are than in here in the spiral arms where there is, metaphorically speaking more room to breath.
Supernovas will fry life in planets for many light years round about whether they occur in the core or spiral arms, but of course there will be more victims in the core because of overcrowding.
Cometary bombardment will be much greater in the core as well because as those stars swirl and occasionally pass even closer to their companions (tenths or less of a light year) they will gravitationally dislodge a lot of "iceteroids" in the Oort cloud of their companion stars turning them into comets and by implication wave after wave of comet.
So the theory goes even though microbiological life may be almost universal (all pun intended) it is being pummeled too often to recover any evolutionary "trajectory" towards complexity. Here on earth where we've been hit a few times we can still recover on the path to complexity. This, not necessarily self destruction, as Carl Sagan assumed, is probably the reason we have not been overrun by members of a single alien species by now.