The shear numbers, which makes for unlimited possibilities.
But those numbers reduce profoundly when one starts considering things like the type star earth has and how uncommon stars like ours are believed to be in the universe.
Finding a star like our sun is not enough because it has to be within the habitable zone of a galaxy. This criteria then wipes out a vast swath of stars in each galaxy.
It doesn't appear that any of the known exoplanets exist in solar systems resembling our own and the composition of our solar system with gas giants in the outer belts trapping interstellar debris is important for life on earth. This then brings up the possibility that our type solar system is itself uncommon which further reduces the probabilities.
On and on the reducing factors go.
There are many, many more reducing factors which Carl Sagan had to know of when he spewed his billions and billions of galaxies containing billions and billions of stars. reasoning for the probability of life out there.