Dr. Seth Shostak, an astronomer with the SETI Institute, points out in his course "The Search for Intelligent Life in Space" what conditions favor the development of life in the universe:

The system's star ("sun") must not be a giant star, because these burn out too quickly before life can fully develop.

The system's star must not be a dwarf star, because such a star locks in the close planets, meaning "one side of the planet forever faces its sun, resulting in horrific weather and unlikely venues for life."

The system's star cannot be a double star, because the unusual gravitational forces created by a double-star sun might not allow stable planetary systems.

The system's star must not be a young star, because stars less than 1 billion years old have not had enough time, so astronomers think, for life to develop.

Ideally, the planet would have a large moon, which creates active tides.

The planet should have tectonic activity, which causes metals to be pushed up to the surface, since metals are valuable to technological civilization.

The planet should have a large planet farther out in its solar system, which by its great gravitational pull cleans the inner solar system of deadly asteroids and comets.

The planet should not have a highly elliptical orbit, which is unsuitable for incubating life.

For life to live on the surface, the planet must have an atmosphere. "Very small planets lose their air, and very large planets tend to sport poisonous atmosphere. Earth-sized planets are ideal."

And it just so happens that all these conditions fit our earth!