How?

How do ghosts become ghosts? Why is it that some people stay behind and others move on and what exactly decides this?
I'm still not sold on the whole ghost's are real deal but I'm getting further proof than I had before. However what makes something stick around as a ghost? Is there some scientific thing at play in regards to "left over energy" or is it something to do with the religious factor? I'm not one that believes the whole god created everything ritual. There are too many variable in this universe at play for me to believe one supreme being created all life. I am stumped though as to why paranormal acts keep happening? What is a ghost and will we ever find out why they are here? there are many questions in this post but they all are ones that we are seeking every day. I think the big reason that I am involved in the paranormal is because I "want" to believe that there is something after the physical world but I am still searching for that answer. Most people believe in a religious afterlife... I don't buy the whole religious aspect but I do want to know, before I am gone, is there something after this life or are we really just here and gone in a blink of an eye.
There is so much we don't know about the universe. We weren't physically around millions and millions and billions of years ago and didn't know that one day we would become a living life form and where exactly were we then? was I an atom? was I some sort of gas? what was I before I was born? these are the questions that I am pondering to this day cause you have to come from somewhere. You don't just magically appear as an egg in your mothers womb... some sort of chemical reaction or particals had to have eventually come together to create that egg. What was it?

I'm stumped on that. Where do we go after this? or is there no after at all and we just become part of the great divide once again never to have existed to anyone but those who knew us. Even then, once they are gone, whos to say we existed at all?

I know, this is all deep, smoke some weed, type stuff here but it has me wondering all these years.