Think about what is required to change the state of a system.
A system in a low energy state requires energy be added from somewhere to bring it to a higher energy state. This therefore can't happen by itself.
However, a system in a high energy state doesn't need external energy to tranform to a low energy state. There may be a higher barrier between the high and low energy states so that a initial push is required. However, once that push occurs, the system emits overall energy. In some cases, this released energy can provide the push for other nearby systems to go from their high to low energy states. This releases more energy, which provides the push to more nearby systems, etc. This is called a chain reaction.
For example, a piece of paper sitting on your desk and the oxygen in the air around it are together in a high energy state. In this case, there is a high enough barrier between the two states that the paper doesn't spontaneously combust on its own. However, providing the initial push by, for example, lighting a match under one corner of the paper causes that corner to transform to a lower energy state. That release heat, which causes more of the paper to transform, etc. Eventually the whole sheet of paper burns up even though you only supplied a small push to one corner.
Now think of the reverse. What mechanism is there for the burnt paper to release its oxygen and return to the original paper state? Even if by some low probability of random states one molecule somewhere did release its oxygen. That absorbed energy overall, so there is now less energy available locally to kick nearby molecules into doing the same.