In my theory, when human died it'll reborn, they'll wake up on other body but without memory, it's just a long dream.
Some other ghosts remain on other dimensions, because it's there decisions.when they decide to go back they'll reborn but if they are tired on Earth they remain on there. The life cycle is hard to explain, our bodies are made up of millions or billions of bacteria but it's good bacteria. Without bacteria there's no life.Imagine the earth and human body are similar, why? It's because, the hair is the same on grass, the moon are the same on eyes, the skin are the on crust and deep down it's the same, the blood look like the hot liquid flames on Earth. The best evidence is the sperms and egg cell, they look like bacteria that combined together to form human body.... Therefore we human are bacteria, it's hard to believe but it's true.