The Second Law of Thermodynamics

If you took a truly random configuration of atoms, there is a vanishingly small probability that it would make anything that can do all the complicated stuff that living organisms can do. And going off of that, if you think about the very specific arrangements of atoms that you need for something like you or me to exist, really, that's a really low probability arrangement. There are so many more ways that you can organize atoms to be a big clump of stuff than to be a human being or a dog or cat.