Imagine you are a stick figure. You have two dimensions and a very limited wardrobe. You decide to walk from point A to point B.
Fig 1: A stick figure contemplating entropy.
Entropy is just a fancy word for "everything eventually becomes a mess." Your room? Entropy. Your hair in the morning? Entropy. The entire universe? Big entropy.
We can't go back because the universe is lazy. It's much easier to break an egg than to put it back together. That's basically physics in a nutshell.