1.)  Stars, that produce elements of all kinds.
These stars should have as much mass as possible so they can reach a high temperature so they can produce heavy elements.
One day these stars will explode and hurl all these elements into the galaxy.

2.) Galaxies that are big enough to collect all these elements and that have a lot of matter where new stars and planets can build. Unfortunately such giant galaxies are rater rare.

3.) A planet on which life can form

4.) Another star that gives the planet life and warmth. The mass of this star should not be too big though, so it’s life span is long enough. This is important, because it takes very long until intelligent life can develop on a planet.

5.) Highly complicated organic combinations like proteins of nucleid acids can build. Simple molecules like carbon hydrogen or single atoms are not able to coordinate biochemical reactions an processes like the metabolism.

6.) There is a certain temperature needed for such complicated molecules to be able to form. Above 100°C such molecules disintegrate and above 1000° only single atoms are still stable. Below 0° the biochemical processes are strongly slowed down so that active life becomes almost impossible.  The ideal temperature would lie between 25°C and 45°C above zero.

7.) To have the right temperature for organic life, the orbit of the planet cannot be too close or too far away. The planet should not have a very eccentric orbit because that would the temperature differences too big.

8.) There should be an atmosphere that blocks most of the UV light.

9.) The combination of elements in the atmosphere doesn’t matter so very much, because living creatures can adapt very good to a lack of oxygen or hydrogen. The surface composition is not so important either, because intelligent life can develop in water as well as on the land. The only thing that would not work would be an entirely gaseous planet.

When one takes a look at this list and all the things that need to be available, life becomes pretty improbable.