Low entropy systems are likely to transition into high entropy systems, and the reverse is unlikely to occur. It is largely a statistical argument; low entropy

Low entropy systems are likely to transition into high entropy systems, and the reverse is unlikely to occur. It is largely a statistical argument; low entropy states are by definition less likely to occur by chance than high entropy states. (You might then ask why we care what happens 'by chance'; the answer is either that physics is fundamentally unpredictable, or that the scale of the universe so large that the influence of the whole on any particular subsystem must necessarily be modeled with noise.)