Einstein always believed that everything is certain, and we can calculate everything. That's why he rejected quantum mechanics, due to its factor of uncertainty. But still quantum physics was right. So why did Einstein never accept the fact of uncertainty? Was there any valid reason that we know of?
Explain it like a story, so it will be interesting.
I am not sure what "explain it like a story" means, but the Bohr–Einstein debates is perhaps the most dramatic story of Einstein making his dissatisfaction known. Pais's scientific biography of Einstein Subtle is the Lord offers an even more comprehensive story of his life and the genesis of his philosophical outlook.
As for the main reason for his opposition, Einstein was pretty explicit about it. He even wrapped it into a quip in a 1926 letter to Born:"God doesn't play dice with the world" (anecdotally, when Einstein repeated the quip during the Bohr–Einstein debates, Bohr's response was "we shouldn't tell God what to do with his dice"). The "God" here is metaphorical, in other words, he felt that physics must provide deterministic and observer-free description of reality. Einstein accepted that quantum mechanics was an experimentally confirmed empirical theory, he simply believed that some of its structural features make it inherently incomplete, as opposed to the view, advanced by Bohr and others, that quantum mechanics is as complete an account of the microphysics as the nature has to offer.
The analogy was to thermodynamics vs statistical mechanics, Einstein felt that quantum mechanics is based on averaged notions, that the uncertainty was an artifact of the description rather than fundamental, and that a more fundamental account of how reality works was in order. He was certainly right on the last part, we are now looking for quantum gravity, but probably not on the idea that it will eliminate the indeterminism he detested. Natarajan's note What Einstein Meant When He Said “God Does Not Play Dice...”, after recounting Einstein's scientific trajectory, gives additional reasons:
"We are now in a position to understand Einstein’s opposition to Quantum Mechanics. This was not the knee-jerk reaction of a person unqualified to speak about physics, but the considered opinion of an eminent scientist based on what he felt were several undesirable features of the theory. First, he was averse to the idea of randomness as a fundamental feature of any theory. He believed that randomness could appear as some form of statistical behaviour but could not be a part of the law, just like a pack of cards that is shuffled according to deterministic laws still shows a random arrangement.
But this was not its only undesirable feature. The theory was also inherently nonlocal, or had a kind of Newtonian action-at-a-distance built into it, but relativity had taught us that all interactions had to propagate at a finite speed. In a landmark paper published in 1935 (twenty years after when Pais considers him finished!), Einstein highlighted the nonlocal and incomplete nature of Quantum Mechanics by proposing the famous Einstein–Podolsky–Rosen (EPR) paradox. The EPR paradox was a gedanken or thought experiment that brought out these undesirable features of Quantum Mechanics, and we all know that Einstein was a past master at thought experiments. Finally, of course, Einstein was opposed to the linear formalism of Quantum Mechanics as an approach to understanding Nature because we have seen above that only a nonlinear theory can contain the equations of motion.
This second part explains why in 1952, when Bohm came up with what Einstein seemingly wanted, a reinterpretation of quantum mechanics which removed the dreaded indeterminism by introducing deterministic hidden variables, the Bohmian particles, Einstein was not taken by it. The problem was that Bohmian mechanics endorsed the "spooky action at a distance" through instantaneous changes in the so-called "quantum potential", which "guides" the Bohmian particles like the de Brogliean pilot wave. Moreover, the Bohmian particles also share the most undesirable property of Lorentz's ether, for which Einstein dispatched it, they are unobservable in principle.
According to Abraham Pais biography of Einstein, Subtle Is the Lord: The Science and the Life of Albert Einstein, and who he knew both personally and professionally, Einstein was not so much bothered by the indeterminancy of quantum mechanics, but by it's non-locality, this was the crux of his paper with Podolsky & Rosen and which goes by the name, the EPR paradox. This is particularly pertinent to Einstein especially, as he discovered the local form of Newtonian gravity, and which Newton himself regarded as a failure of his theory - that it acted at a distance. This is General relativity. Here it is spacetime itself that acts as the bearer of the metric field and which is gravity.
