"You need a sun of the right size and composition<br>You need your sun to be in the habitable zone of its galaxy where there are the right elements and not too much radiation<br>You need the planet to be in the "Goldilocks" zone of its star<br>You need the planet to be large enough to hold onto its atmosphere but not much larger or you'll have too much gravity for tree-like plants or animals with upright skeletal structures.<br>You need a moon to stabilize the rotation"," and not too big or too small.<br>You need a moon to create tides so life can move over millennia from the oceans to land.<br>You need a tilted axis (but not too tilted) to create seasons and temperate zones", otherwise," you have a burning desert sandwiched between ice caps.<br>The previous three items mean you need the plant to collide with another planetoid of the right size and in the right spot<br>You need gas giants to sweep out all the loose rock and ice or the planet gets impacted too often for life to evolve<br>You need the gas giants (after cleaning the sky) to politely move out to orbits too far out to cause destructive tidal stress or an overly elliptical orbit<br>The life that emerges must have intelligence and fine motor skills to create technology. Dolphins could be as smart as humans but cannot ever use fire or build machines.<br>You need all of these things to happen at exactly the right times and in the right order"," over billions of years.<br>So", it may be that it's really, really, really rare to have the right conditions for life to emerge and survive to a technological stage of development, and it may impossible for the civilizations that emerge to ever communicate or even be aware of others due to the distances involved, both in space and in time.