Einstein added the cosmological constant lambda to general relativity to keep the universe from expanding. We guess at his intuition.
Inspired by this video: Einstein's Famous Blunder, which makes no attempt to explain why Einstein was uneasy with a non-static universe. I don't think it should be considered a blunder.
In a nutshell, we guess that Einstein's intuition was the mediocrity principle / Copernican principle. According to the principle, the time point that we live in is not special within the lifespan of the universe.
If the universe is expanding, then things will become more and more spread out. However, when we look at the skies, especially distant galaxies, we can imagine things being much, much more spread out. We do seem to be living at a special time point in the universe when we can still see distant galaxies, their light not yet having been redshifted to invisibility. This violates the Copernican principle. By the principle, we should only be able to see our gravitationally bound Local Group.
Even slightly applying the anthropic principle doesn't help: that principle only requires that some intelligent life observe the universe, not necessarily intelligent life like us. The black hole era of the future universe is a much larger block of time than the current stelliferous era. It would have been much more likely for us to be intelligent life feeding on Hawking radiation, observing the (boring) universe and applying the mediocrity principle.
We could reason further, that intelligent life must cease, become impossible, in another 14 billion years or so.
The problem Einstein was trying to solve with the cosmological constant remains a problem, so his solution, while incorrect for setting up a static universe, shouldn't be considered a blunder: Why is it that we seem to be living in a very special time point in the universe? Why is the universe we observe not representative of the most likely states of the universe?
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