Earth is the right distance from the Sun, basking in the rather narrow habitable zone. Mars and Venus, although reasonably close to us, did not fare so well.
Earth has the liquid water needed for life near the surface and the oceans are just the right size, not too large and not too small. Larger oceans would have covered the land provided for complex evolution while smaller oceans would not have allowed ocean circulation belts to average global temperatures and salinity.
Earth has a large moon, thanks to a freak collision of Earth with an object the size of Mars. Had it hit Earth directly, both bodies would have shattered. If it had hit at a more glancing angle there would be either no moon or a much smaller one.
The Moon, which happens to be the right distance away, stabilizes Earth's tilt and creates the tides that nurture life in marshes and estuaries. A near perfect tilt has kept the seasons from being too severe.
Earth is far enough from the Sun to avoid locking the Moon's tides.
Earth is the right mass so that its gravity retains the atmosphere and oceans but does not flatten living organisms. If it were much stronger, only crickets would survive.
Earth has a solid/molten core that provides enough heat for plate tectonics, as the elevated landmasses essentially "float." The benefits of this are explained later.
Earth has a carbonate-silicate cycle that holds its temperature in bounds. (Keep in mind that normal temperature cycles for millions of years have swung from lengthy ice ages to brief interglacial periods like the present, which is somewhat extended.) The geochemistry of this cycle is a little complex and can be found at the following website.
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