Earth supports life because it is the correct distance from the sun, its composition is rock, it has a molten core, it gains protection from its atmosphere and it has water. There are approximately 500 planets in the Milky Way capable of supporting life.
Earth's distance from the sun ensures that it receives appropriate warmth and light, without being so close as to become uninhabitable. Sunlight is a necessary source of energy for all life on Earth as it drives photosynthesis that keeps producers alive. Earth's rocky crust provides a stable surface for life, unlike gas giants, such as Jupiter or Saturn.
Just as important as Earth's rocky crust is its molten core. Earth's core provides a source of geothermal heat and energy as well as generating a magnetic field that protects Earth from solar storms. Earth also gains protection from its atmosphere, which keeps out excessive radiation. Finally, Earth has water. Besides being vital to life on Earth now, water is the catalyst that allows initial life to develop.
They diversity of life on Earth today is the result of the age of both the Earth itself and the sun. A habitable planet must orbit a star capable of surviving several billions of years so that a multitude of life has the chance to grow.