According to the second law of thermodynamics, the entropy of a closed system tends to increase. It was Boltzmann who provided the correct definition of entropy. He also explained why the second law of thermodynamics was true. His definition of entropy was based on the number of arrangements of atoms or molecules in a system. He showed that increasing entropy was natural result of the fact that increasing entropy was statistically more probable than decreasing entropy.

The same argument holds for why heat energy flows from the hot to the cold side. This happens because flowing from hot to cold is highly statistically probable than flowing in the reverse direction. Though the flow in the reverse  direction is very much plausible, the extremely low probability of this thing happening makes it not occurring in reality.

Just to illustrate this point, let us consider an example. For instance, think of a deck of shuffled playing cards in its pristine rank-and-suit precise order.

As you shuffle the deck, does randomness increase or decrease? Will the deck ever end up back in its pristine rank-and-suit precise order as you shuffle the deck repeatedly? It is extremely unlikely. The probability that it will end up that way is extremely low, whereas the probability of getting increasingly random is extremely high.

The point is, as you shuffle more, what is happening is that the randomness of the deck increases because it is (statistically) highly probable than ending up in a pristine state. That is the reason you will never see the pristine state even if you shuffle the deck whole of your lifetime!