This is a great question.
The question of galactic goldilocks zones is covered in the book Rare Earth:
Rare Earth: Why Complex Life is Uncommon in the Universe
Highly recommended reading.
For complex life its necessary to have (but not limited to) stability and the right chemical elements. The elements are Carbon, Hydrogen, Nitrogen, Oxygen, Sulphur and Phosphorus. All but Hydrogen are produced in stars (phosphorus being the least abundant) So to get the "metallicity" of a planet up, it needs to include the ashes of a few generation of older stars. As you move further out towards the outer edges of a galaxy star formation rate decreases and at some point is too low for these elements to be available.
As you move further in towards the core of the galaxy star density increases and this reduces the environmental stability around the planetary systems formed in these regions. More chances of nasty things happening that can sterilize a planet or eject it from its orbit.
So too far out and you don't get the chemical elements and too far in and you don't get a stable environment. There is your goldilocks zone.
Another thing to think about is the type of star your planet formed around. The goldilocks zone around a star, the orbits where a planet has liquid water on the surface, moves as the star ages. Stars get brighter as they age so the goldilocks zone moves slowly out over the life of a star. We humans took about 4 billion years to evolve so lets accept that to have intelligent life on other planets you need that planet to be in the goldilocks zone for at least 4 billion years continuously. When the earth formed the sun was 30% dimmer than today but fortunately the earth formed on the outer edge of the goldilocks zone around the young sun. As the sun aged the goldilocks zone moved out and we are currently nearing the inner edge of the zone. We'll be outside the goldilocks zone in about 1 billion years. More than enough time for an intelligent species to evolve.
However, if the sun was 10% larger it would increase in brightness as it aged faster and there would be no orbit that stays inside the zone for 4 billion years, so no time for intelligent life to evolve. If the sun was 10% smaller the zone is also smaller and there is still no orbit that stays inside for 4 billion years, so again no time for intelligent life to evolve.
So a planet needs to form in the right zone in the galaxy and in the right orbit around the right sized star for just a chance of intelligent life to exist.
Lucky us.
Chris