The "arrow of time", which points from the past towards the future, is thought to have its origins in the Big Bang. The infant Universe is thought to have had very low entropy – a measure of disorder or randomness. Since then, entropy has been increasing – this change is what gives the arrow of time its directionality. It's the same reason it's easy to crack a fresh egg, but extremely hard to take a mess of shards and yolk and create a whole fresh egg.