The opening paragraph of the section the Introduction to Standard Big-Bang Model in the PDG review claims that
The observed expansion of the Universe is a natural (almost inevitable) result of any homogeneous and isotropic cosmological model based on general relativity.
Why is the expansion of the Universe claimed to be natural and inevitable? In other words, why would a static Universe be unnatural?
A static universe is not viable in GR. It requires a precarious fine tune of the total mass density and the cosmological constant. Furthermore, this balance is unstable. Any deviation from it will either cause the universe to start an accelerated expansion, or start contracting towards a big crunch. This is why Einstein called his introduction of a cosmological constant to allow for a static solution, his biggest blunder.
So we are left with the option of either an expanding or contracting universe. Both could have been the case. We could also have had scenarios where expansion eventually turned into contraction. Instead we (surprisingly) find that the universe experiences accelerated expansion.
