Why is Earth able to support human life?

Life on Earth

Life on Earth is possible because of a coincidental combination of factors:
  • The habitable zone: Earth is at the right distance from the Sun to have liquid water on its surface, and for the atmosphere to be at a reasonable temperature for life to occur. It also doesn't stray too far from the optimal distance, making the temperature stable.
  • Stable planet: we aren't inundated with lava every day. This gives life the chance to happen without being destroyed instantaneously.
  • Water: we have it. necessary for life.
  • Atmospheric composition: began with plenty of carbon oxides and simple organic compounds with very little free oxygen; an important atmosphere for abiogenesis.

Plenty of other factors matter as well.

Life only on Earth

Earth is currently the only planet known to support life in any form. Because of the existence of millions or billions of other planets, and the fact that we have not - indeed, cannot - inspect all of them means we will never know for sure the exact number that do support life. Many have one or two or some of the necessary conditions for life, but none have so far been discovered (of the few hundred we know) that have all of the necessary factors.

Life only on Earth

Some good points have been mentioned here but there are literally hundreds of things that make Earth just right for life. Here is a few to add:
  • Axial tilt. Our planet has a reasonable and more importantly quite stable tilt to it (around 23-24 degrees off axis). This lets the weather be relatively stable and thus becomes a candidate for life.
  • Composition of the planet and its atmosphere. We need water, carbon, hydrogen, oxides (like oxygen gas and ozone), carbon oxides, and nitrogen gas and more.
  • An iron core; our planet would be harmed by the suns radioactive rays without an iron core to protect us (mostly).
  • The planet had to be stable for at least 100 million years for life to form - most planets would not follow this.
  • We need a moon that is about 1/3 the size of Earth to regulate our orbit.
  • Earth's path has to be a good ellipse around the sun.
  • Earth has to be the right size and not moving too fast.
  • and and