The truth is there are a lot of things that are present in the earth that humans need which are totally absent in other planets. One important thing is the intensity of sunlight that we receive. Earth is placed in a distance that humans get the enough temperature we need from the Sun. Imagine if Earth is put in Mercury.

Second is the presence of our atmosphere. Earth is only the planet that has the atmosphere suitable to humans. With its layered structure, without it the earned should've been dead.

Third is the presence of oxygen that human needs. Oxygen is the most important element for our breathing and that without it, humans simply can't exist.

Fourth, is the presence of freshwater which is the most suitable liquid that helps our body circulate and activate. 

Lastly, planet Earth has the ability to cycle everything so that it is sustainable. A cycle and process are done in all matter, that's why Earth is still living and functioning as a planet suitable for humans.