I wrote this up in a longer form once - but I can't find it so here is a bit of it from memory.
If you accept basic scientific theories and facts a lot of things needed to happen just right in order for us to exist on this planet.
1. The big bang happened for some reason.
2. As the universe is expanding and cooling particles clump together to form larger particles. Eventually these join other particles to for small rocks and then planets.
3. Our planet gets caught in the gravitation field of a nearby star in just the right place. If we were closer it would be too hot for life - further away and it would be too cold.
4. Our planet just happened to be carbon based which is lucky for us because all life we have observed is carbon based.
5. In the early days of the planet we were bombarded by meteors and asteroids. Life could have never formed in that hostile environment. Luckily, a huge chunk of our planet was expelled when a large body struck it. This new body orbits the earth (actually they're dual planets) and acts as a shield against much of the space debris that would hit the earth.
6. Not to mention Saturn and Jupiter that act as a first line of defense effectively sweeping the solar system clean.
7. Since the earth is cooling gases start to form so level of an atmosphere. Eventually this gives rise to lightening.
8.
This lightening strikes just the right combination of biochemicals that are present to form the first bits of life.
9. Life dies and forms again. Survival is difficult. Eventually more complex life forms arise.
10. Evolution takes hold and those species that are most able to adept to the quickly changing planet multiple and pass on their genes.
11. Since this is a winner takes all environment - those species which are the most fierce survive at a greater rate. Survival of the fittest is born.
12. Eventually this gives rise to the dinosaurs. Huge fierce beasts that prey on nearly everything around them.
Time for a pause - man (as we know it) could not have co-existed with the dinosaurs. Upright walking man would have been quickly eaten by the faster and bigger dinosaurs.
13. A huge asteroid hits earth killing the dinosaurs. Now the planet is ready for a new species to climb up the evolutionary ladder.
14. Intelligent life (as we know it) begins to evolve.
Now some of the details might be a bit off but the basic idea is that while there are lots of planets where the conditions might be right for life - a lot of luck went in to intelligent life evolving on this planet.
If you look at each of the 14 points above having some level of probability of happening then the odds of all of them happening gets pretty small. Granted, that over enough planets and enough time the likelihood increases.
There is some talk about a 'Great Filter' - that is some event that must happen before intelligent life can communicate with intelligent life on another planet. I find the idea fascinating and if you're interested there is a wonderful article on the subject at this link:
http://www.centauri-dreams.org/?p=1848