Why I believe it is almost impossible to find complex intelligent life outside Earth.
There's a huge singular event that made complex high-metabolic life possible on Earth that can be summarized as "Amoeba ate a bacteria called 'Mitochondrion', but didn't digest it."
The excrement from that Mitochondrion turned that amoeba from a snail into a cheetah, reproducing and depleting resources from the slow-paced competition, with each new generation growing with their own sets of "domesticated" mitochondria and destroying all competing life forms.